Islamic Fiqh > Marriage and family > Marriage impediments > Combining two sisters, by blood or by breastfeeding, forbidden by consensus
Contents
Updated on 25 September 2026 at 3:23 PM
This part belongs to the full section: All sections of this part.
This second part on the women forbidden in marriage deals with women forbidden by combination (jam'): a man may not combine in marriage two sisters, or a woman with her paternal or maternal aunt, while other combinations remain lawful (a woman with her cousin, a wife with her stepdaughter from another man). It then covers the practical cases (one contract or two contracts, unknown order), and then the second category of prohibition: the one due to the right of another, namely the already-married woman and the woman in her waiting period, together with the status of the woman he fornicated with.
The principle that opens this part: it is likewise not lawful to combine a woman with her maternal aunt. If we supposed one of the two to be a male, the maternal aunt would become a maternal uncle, and a maternal uncle may not marry the daughter of his sister. If we supposed the girl to be a male, the other would be his maternal aunt, who is not lawful to him. The same applies to every forbidden combination.
It is not permitted to combine two sisters, whether by blood, by breastfeeding, or one by blood and the other by breastfeeding, in a marriage contract, and this by consensus of the Muslims. The source is the word of Allah:
and to combine two sisters (in marriage)
Surah An-Nisa, 23
The generality of the wording entails the prohibition of combining in all cases. Combining the two leads to severing the ties of kinship: enmity between the two rivals is evident, and severing the tie is unlawful; so is what leads to it ('Bada'i' as-sana'i'', 2/262).
The hadith of Umm Habibah illustrates this. She said to the Prophet (peace be upon him):
O Messenger of Allah, marry my sister, the daughter of Abu Sufyan. He said: Would you like that? She said: Yes, for I am not your only wife, and my sister is the dearest to me of those who would share the good with me. The Prophet (peace be upon him) said: That is not lawful for me. She said: We were talking among ourselves that you intended to marry Durrah bint Abi Salamah. He said: The daughter of Umm Salamah? She said: Yes. He said: By Allah, even if she were not in my fosterage of suckling, she would not be lawful to me: she is the daughter of my brother through suckling, Thuwaybah suckled Abu Salamah and me. So do not present to me your daughters nor your sisters.Reported by Al-Bukhari (4818)
Imam Ibn Battal (may Allah have mercy on him) said: the scholars agreed that it is not permitted to combine the marriage of two sisters in a single contract, by the verse cited and by the saying of the Prophet (peace be upon him): 'Do not present to me your sisters' ('Sharh Sahih al-Bukhari', 7/214-215).
As for combining through possession (milk al-yamin), all the scholars hold that it is not permitted as regards intimacy (wat'), even though combining them in possession is permitted: intimacy with slave women parallels the marriage contract with free women. Only the Zahiris dissented, permitting combining in intimacy as in possession.
Ibn al-Mundhir (may Allah have mercy on him) said: the people of knowledge agreed that the contract combining two sisters is not permitted ('al-Ijma'', 367; 'al-Ishraf', 5/99). Ibn Abd al-Barr: they agreed on the prohibition of contracting with the sister of the wife, the verse referring to the two wives by marriage contract ('al-Istidhkar', 5/490). Al-Qurtubi: the community agreed on the prohibition of combining them in a single marriage contract, by this verse and by the saying of the Prophet (peace be upon him): 'Do not present to me your daughters nor your sisters' ('Tafsir al-Qurtubi', 5/116).
Al-Mawardi specifies: combining the two sisters is unlawful by the text of the Book and the consensus of the community. As for combining through possession, if one combines them in possession by purchase, it is permitted as long as one does not combine them in enjoyment: the purpose of possession is to prevent enjoyment, hence the lawfulness of owning sisters or aunts whose intimacy is not lawful, unlike the marriage contract whose purpose is enjoyment. The marriage is therefore void in case of combination, while possession is not. If he wants to combine them through possession in order to enjoy each of them, it is not permitted: this is the view of the generality of the Companions, the Successors and the jurists ('al-Hawi al-Kabir', 9/201).
Ibn Rushd: they agreed that the two sisters are not combined in a marriage contract, by the word of Allah cited. They differed on combining through possession: the jurists forbid it, a group permits it. The cause of their disagreement is the confrontation with the generality of the word of Allah and the exception at the end of the verse: 'except those your right hands possess' (Surah An-Nisa, 24). This exception may refer to the nearest item mentioned, or to everything the verse made unlawful apart from what consensus excludes: possession would then fall outside the generality of the word of Allah. Or it refers only to the nearest item mentioned: the word of Allah then keeps its generality, especially if we give the prohibition the cause of sisterhood or a cause present between them ('Bidayat al-Mujtahid', 2/31). Those who forbid when one is married and the other owned are Malik and Abu Hanifah; Ash-Shafi'i permits it.
The Muslims also agreed on the prohibition of combining two sisters by suckling, or a sister with the suckling-sister of the other, by the word of Allah cited. Ibn Abd al-Barr illustrates: if a man marries two infant girls, then a woman comes and suckles both, they become sisters by suckling, both are forbidden to him, and he resumes the contract of whichever he wants ('at-Tamhid', 17/213).
The credible scholars agreed that it is not permitted for a man to combine a woman with her paternal aunt or her maternal aunt, by blood or by suckling, according to the hadith of Abu Hurayrah (may Allah be pleased with him):
The Messenger of Allah (peace be upon him) forbade that a woman be married over her paternal aunt or her maternal aunt.Reported by Al-Bukhari (4891) and Muslim (1408)
In another wording:
A woman is not married over her paternal aunt, nor the paternal aunt over her brother's daughter; a woman is not married over her maternal aunt, nor the maternal aunt over her sister's daughter; the elder is not married along with the younger, nor the younger with the elder.Authentic hadith: Abu Dawud (2065), at-Tirmidhi (1125), Ibn Hibban in his Sahih (4118)
At-Tirmidhi (may Allah have mercy on him): the general practice of the scholars, with no known disagreement among them, is that it is not lawful for a man to combine a woman with her paternal or maternal aunt. Whoever marries a woman over her aunt, or the aunt over the niece, has the marriage of the other dissolved (mafsookh). This is what the generality of the people of knowledge say ('Sunan at-Tirmidhi', 3/433).
Ibn Abd al-Barr: the scholars agreed on the soundness of this hadith. Among them it is not permitted for a woman to be married over her paternal aunt, however high in degrees, nor over her sister's daughter, however low, nor over her maternal aunt, however high, nor over her brother's daughter, however low; suckling is like blood. Some hadith scholars had claimed that no one narrated it except Abu Hurayrah, whereas it was narrated by Ali ibn Abi Talib, Ibn Abbas, Ibn Umar, Abdullah ibn Amr ibn al-As and Jabir ('at-Tamhid', 18/277; 'al-Istidhkar', 5/452, 455). Ibn al-Mundhir confirms the consensus: neither a woman over her aunt, nor the elder with the younger ('al-Ijma'', 370; 'al-Ishraf', 5/100-101).
Al-Jassas: what the Book prohibits textually is combining the two sisters; recurrent (mutawatir) reports came prohibiting the combination of a woman with her paternal or maternal aunt, narrated by Ali, Ibn Abbas, Jabir, Ibn Umar, Abu Musa, Abu Sa'id al-Khudri, Abu Hurayrah, Aisha and Abdullah ibn Amr: the Prophet (peace be upon him) said: 'A woman is not married over her paternal aunt, nor over her maternal aunt, nor over her brother's daughter, nor over her sister's daughter', and in some reports: 'nor the younger with the elder, nor the elder with the younger', with variations of wording and agreement of meaning. The people received them with acceptance for their recurrence and diffusion; they are reports that necessitate knowledge and action, and their ruling must therefore be applied along with the verse.
A group of Khawarij dissented, permitting combination apart from the two sisters, invoking the word of Allah:
lawful to you are all beyond those mentioned
Surah An-Nisa, 24
They erred in this and strayed from the right path, for Allah, while saying this word, also said: 'Whatever the Messenger gives you, take it' (Surah Al-Hashr, 7). The prohibition of these combinations is established from the Prophet (peace be upon him), so it must be annexed to the verse, which remains operative for what lies beyond the two sisters and beyond those the Prophet (peace be upon him) specified ('Ahkam al-Qur'an' of al-Jassas, 3/79).
Ibn Battal: the scholars agreed that it is not permitted to combine a woman and her paternal aunt, however high, nor the woman and her maternal aunt, however high ('Sharh Sahih al-Bukhari' of Ibn Battal, 7/216). Al-Kasani adds: combining a woman with her daughter is unlawful like combining the two sisters, and even stronger, for the kinship of birth is unitive without dispute; severing the tie is unlawful, and so is what leads to it ('Bada'i' as-sana'i'', 2/262).
The disagreement concerned the other combinations of relatives forbidden to each other (if one were a male, the marriage of the other would be forbidden to him from both sides), such as combining a woman with her paternal or maternal aunt. The generality of the scholars say: it is not permitted. Uthman al-Batti said: combination apart from the two sisters and apart from a woman with her daughter is not unlawful, invoking the word of Allah: 'lawful to you are all beyond those mentioned' (Surah An-Nisa, 24): the prohibitions were mentioned, among them the combination of the two sisters was forbidden and the rest made lawful; combination apart from the two sisters therefore does not enter the prohibition but enters the lawfulness. The combination of a woman with her daughter is forbidden by indication of the text, the kinship of birth being stronger. The answer: the famous hadith narrated from Abu Hurayrah announces that whoever marries the paternal aunt and then her brother's daughter, or the maternal aunt and then her sister's daughter, may not do so; then it announces that if he marries the niece first and then the aunt, it is also not permitted, lest one think that the prohibition of combination is limited to one of the two orders, like the marriage of the slave woman over the free woman, which is not permitted, whereas the marriage of the free woman over the slave woman is permitted. And because combining two relatives forbidden to each other in marriage causes the severing of ties: the two rivals dispute and quarrel and do not get along, something known by custom; this leads to severing the tie, which is unlawful, and marriage is its cause, so it is forbidden so as not to lead to it. The Prophet (peace be upon him) alluded to this meaning at the end of the hadith in some narrations: 'If you did that, you would cut off their kinship ties', or: 'they would cut each other off', or: 'it entails severance'. Anas (may Allah be pleased with him) narrated: 'The Companions of the Messenger of Allah (peace be upon him) disliked combining relatives in marriage and said: it begets grudges.'
Abdullah ibn Mas'ud disliked combining the two daughters of two paternal uncles and said: 'I do not declare it unlawful, but I dislike it.' The dislike comes from the possible severance; the absence of prohibition comes from the fact that their kinship does not entail the tie. As for the verse, 'beyond that' means beyond what Allah forbade; yet the combination of a woman with her aunt or her daughter was forbidden by Allah on the tongue of His Messenger, which is revelation not recited. Moreover, the prohibition of combining the two sisters has a cause (the severing of the tie), and this combination leads to the severing of the tie: it is therefore a prohibition fixed by the text, and it is not 'beyond' what was forbidden in the verse of prohibition ('Bada'i' as-sana'i'', 2/262-263).
Ibn Rushd: they agreed on the prohibition of combining a woman with her paternal or maternal aunt, established from the Prophet (peace and blessings be upon him) by the hadith of Abu Hurayrah and its recurrence: 'A woman is not combined with her paternal aunt nor with her maternal aunt.'
The definitions come from Ibn Rushd ('Bidayat al-Mujtahid', 2/31): the paternal aunt ('ammah) is every female who is the sister of a male whose offspring attaches to you, directly or through another male; the maternal aunt (khalah) is every female who is the sister of a female whose offspring attaches to you, directly or through another female; and they are the grandmothers on the mother's side.
Al-Mawardi: combining a woman with her paternal or maternal aunt is unlawful, in the marriage contract as in possession, like combining the two sisters: this is the view of the majority. It is narrated from the Khawarij and from Uthman al-Batti that neither marriage nor possession forbids it; Dawud (the Zahiri) forbade it in marriage but not in possession. As for Dawud, the discussion with him already took place regarding the two sisters. As for al-Batti and the Khawarij, they argued: the prohibition of marriages is taken from the text of the Book alone, not from the Sunnah, and the Book did not come with that, so there is no prohibition. This is an error: everything the Sunnah brought must be applied just as what the Book brought, Allah having said:
He does not speak from desire. It is nothing but a revelation revealed.
Surah An-Najm, 3-4
And the Sunnah came with what Malik narrated from Abu'z-Zinad, from al-A'raj, from Abu Hurayrah: 'A woman is not combined with her paternal aunt nor with her maternal aunt'; and Dawud ibn Abi Hind narrated from ash-Sha'bi, from Abu Hurayrah, that the Prophet (peace be upon him) said: 'A woman is not married over her paternal aunt, nor the paternal aunt over her brother's daughter; a woman is not married over her maternal aunt, nor the maternal aunt over her sister's daughter; the younger is not married with the elder, nor the elder with the younger.' These two hadiths are explicit texts, and the second is more complete. Although they are a single report, the community received it with acceptance and the majority acted upon it, so that it became like the recurrent reports; the Khawarij must therefore act upon it even if they do not accept solitary reports. And because the two sisters may not be combined: if one were a male, her sister would be forbidden to him; likewise the woman with her paternal and maternal aunt: if one were a male, his paternal or maternal aunt would be forbidden to him ('al-Hawi al-Kabir', 9/204).
Ibn Qudamah raises the issue, then reports from Ibn al-Mundhir: the people of knowledge agreed on this ruling, with no disagreement, except for some innovators whose opposition does not count as disagreement, namely the Rawafid and the Khawarij, who did not declare it unlawful and did not follow the Sunnah established from the Messenger of Allah (peace be upon him). If they invoke the generality of 'lawful to you are all beyond those mentioned' (Surah An-Nisa, 24), we specify it with what we have narrated ('al-Mughni', 7/88).
It is reported that two Kharijite men came to Umar ibn Abd al-Aziz and objected to him regarding the stoning of the fornicator and the prohibition of combining a woman with her paternal aunt or her maternal aunt, saying: 'This is not in the Book of Allah.' He asked them: how many prayers did Allah prescribe for you? They said: five prayers in the day and the night. He asked them the number of rak'ahs and they told him; he asked them the amount of zakah and its rate and they told him. He said: Do you find that in the Book of Allah? They said: we do not find it in the Book of Allah. He said: From where did you get it, then? They said: the Messenger of Allah (peace be upon him) did it, and the Muslims after him. He said: 'It is the same for this.'
There is no difference between the maternal aunt and the paternal aunt, literally or figuratively: the aunts of the fathers and the aunts of the mothers, however high their degrees, whether by blood or by suckling. Any two persons, one of whom could not marry the other if one were a male and the other a female because of kinship, may not be combined in marriage: this leads to the severing of close kinship, rivalry and jealousy between rivals being innate. It is not permitted either to combine a woman with her mother in the contract, for what has been said; and the mother is even closer to her daughter than the two sisters are to each other: if the two sisters are not combined, the woman and her daughter are more deserving of that ('al-Mughni', 7/88).
The jurists of the four schools agreed that it is permitted to combine a woman with the daughter of her paternal aunt, her paternal uncle, her maternal aunt or her maternal uncle: if one of the two were a male, he could marry the daughter of his paternal uncle, his paternal aunt, his maternal uncle or his maternal aunt. This is the foundation of the prohibition and the lawfulness of combinations among relatives; by this meaning we forbid combining a woman with the paternal aunt of her father or her mother, or with the maternal aunt of her father or her mother: if one were a male, the marriage of the other would be forbidden to him.
Ibn Abd al-Barr narrates: Ma'mar narrated from Qatadah that there is no harm in a man combining the two daughters of the paternal uncle. Abu Umar (Ibn Abd al-Barr) adds: on this view stand the majority of the scholars and the body of the jurists, the imams of fatwa: Malik, Ash-Shafi'i, Abu Hanifah, ath-Thawri, Ahmad, Ishaq, al-Awza'i and others ('al-Istidhkar', 5/454; 'Bidayat al-Mujtahid', 2/32; 'al-Hawi al-Kabir', 9/204).
Ibn al-Mundhir opens the chapter of combining the daughters of the paternal uncle with the word of Allah:
So marry what pleases you of women, two or three or four.
Surah An-Nisa, 3
The scholars differed on combining the daughters of the paternal uncle. The majority of the people of knowledge permitted it: al-Hasan al-Basri, al-Hasan ibn al-Husayn ibn Ali, al-Awza'i, Ash-Shafi'i, Ahmad, Ishaq, Abu Ubayd and the generality of the people of knowledge. 'Ata' disliked this combination, as did Jabir ibn Zayd and Sa'id ibn Abd al-Aziz.
The jurists of the four schools and others stipulate that it is permitted for a man to combine the wife of a man and her daughter from another man (which presupposes that he has not consummated his marriage with the mother, otherwise the stepdaughter would be one of his mahram by marriage), since there is between them neither kinship nor suckling.
Abu Bakr said: the marriage is valid if he combines them, and I know of no one who invalidated this marriage ('al-Ishraf', 5/103). Abu Umar (Ibn Abd al-Barr): the scholars differed on the man combining in marriage the wife of a man and her daughter from another. The majority hold it valid, as do the jurists of Madinah, Makkah, Iraq, Egypt and Syria, except Ibn Abi Layla of Kufah, preceded in that by al-Hasan, Ali and 'Ikrimah; most of the jurists contradicted them, since there is no kinship between the two ('al-Istidhkar', 5/454-455; see 'Bidayat al-Mujtahid', 2/32; 'Mukhtasar ikhtilaf al-ulama', 2/308; 'al-Jawharah an-Nayyirah', 4/275; 'Umdat al-Qari', 20/107).
Al-Mawardi reports the saying of Ash-Shafi'i: 'There is no harm in a man combining a woman and her father's wife, and the wife of a man and the daughter of his wife from another: there is no kinship between them.' Al-Mawardi judges this correct: a man may combine a woman and his father's wife, and his son's wife: this is the view of the majority of the people of knowledge, except Ibn Abi Layla who forbade it, arguing that they are two women one of whom, if she were a male, would find the other forbidden, for she would be his father's wife or his son's wife; hence their combination would be forbidden like the combination of a woman with her paternal or maternal aunt, for the meaning mentioned. This is an error: it is reported that Abdullah ibn Ja'far ibn Abi Talib and Abdullah ibn Safwan ibn Umayyah each combined the wife of a man and her daughter from another, without any scholar of their time objecting, which amounts to consensus. The prohibition of combination is only established between relatives, to preserve the ties of kinship and prevent them from being cut by mutual hatred and disobedience; between these two there is neither kinship nor suckling subject to the ruling of kinship, so the combination is not unlawful, as with other unrelated women ('al-Hawi al-Kabir', 9/212).
Ibn al-Mundhir devotes a chapter to combining the wife of a man and her daughter from another by marriage. The scholars differed; most of the people of knowledge permitted this marriage, and Abdullah ibn Ja'far and Abdullah ibn Safwan ibn Umayyah practiced it. It was permitted by Muhammad ibn Sirin, Sulayman ibn Yasar, ath-Thawri, al-Awza'i, Ash-Shafi'i, Ahmad, Ishaq, Abu Ubayd, Abu Thawr and the people of opinion; Malik said: 'I do not know it as unlawful.' We say: I find no proof prohibiting the combination, and the word of Allah 'Marry what pleases you of women' (Surah An-Nisa, 3) makes lawful the marriage of all women except those forbidden by the Book, the Sunnah or agreement, and 'lawful to you are all beyond those mentioned' (Surah An-Nisa, 24) except what the Sunnah forbade. It is reported that al-Hasan al-Basri and 'Ikrimah disliked it; as for al-Hasan, his retraction is established, and the isnad of the report of 'Ikrimah is defective ('al-Ishraf', 5/102).
Ibn Qudamah: 'There is no harm in combining the one who is the wife of a man and her daughter from another.' Most of the people of knowledge consider lawful the combination of a woman and her stepdaughter; Abdullah ibn Ja'far and Abdullah ibn Safwan ibn Umayyah practiced it, and the rest of the jurists say the same, except al-Hasan, 'Ikrimah and Ibn Abi Layla whose dislike is reported, because if one were a male, the other would be forbidden to him, resembling the woman and her aunt. Our proof: the word of Allah 'lawful to you are all beyond those mentioned' (Surah An-Nisa, 24); there is no kinship between them, so they resemble two unrelated women; and combination is forbidden only out of fear of severing close kinship between paired persons, and there is no kinship between these two ('al-Mughni', 7/98).
In closing the case of doubt between two contracts, the Hanafis: the one who is in doubt keeps the one he retains, with half of the named dower; the consummation of the second obliges the dower of equivalence only for the one consummated; if the first contract is in doubt, both marriages are dissolved, each has against the one who consummated with her her dower of equivalence, no marital claim of one upon the other is established, each of the two women has against the other half of the named dower, and this right does not fall because of the doubt ('al-Mughni', 7/98-99).
The marriage of a man with the two women forbidden by combination, like two sisters or a woman with her paternal or maternal aunt, by blood or by suckling, is of two types. First type: he contracts with both in a single contract: their marriage is void and the contract is not valid on either of them, for it is not possible to validate it on one of the two nor to prefer one over the other; it is void for both, since one cannot distinguish the one to whom validity would belong, like the woman married to two men.
The Shafi'is and the Hanbalis: it is the same whether he consummated with one or not. He has the choice to renew the contract with whichever he wants: if he contracts with the one he consummated with, the waiting period due to his intimacy with her falls away; if he contracts with the one he did not consummate with, his contract is valid, and it is recommended to abstain from intimacy with her until the end of the waiting period of her sister due to his intimacy, so that his fluid does not gather in two sisters ('al-Hawi al-Kabir', 9/205; 'al-Mughni', 7/94; 'Sharh az-Zarkashi', 2/375; 'al-Mubdi'', 7/63; 'al-Insaf', 8/124; 'Sharh Muntaha al-iradat', 5/163-164; 'Kashshaf al-qina'', 5/82; 'Matalib uli an-nuha', 5/199).
The Hanafis: if he marries two sisters in a single contract, he is separated from them. If this occurs before consummation, they have nothing; after it, each has the lesser of her dower of equivalence and her named dower; then he may not marry either of them until the waiting period of the other has ended ('al-Jawharah an-Nayyirah', 4/274).
The Malikis: if he marries two women forbidden by combination in a single contract, the marriage is dissolved without divorce. If he marries them in two contracts, the marriage of the second of the women forbidden by combination is dissolved without divorce, with no dower for her if the dissolution occurs before consummation, when she confirms being the second or evidence is found. If she does not confirm it and claims to be the first without evidence, or says 'I do not know', her marriage is dissolved by divorce, acting upon her statement; the husband swears that she is the second and not the first, in order to drop the half of the dower due by divorce before consummation; if he retracts, he owes her half of the dower by his mere retraction. If this is discovered after consummation, the marriage is dissolved by divorce, the full dower is due to her by consummation, no oath is taken against him, and he remains on his first marriage by his claim, without renewal of the contract.
The second of the two forbidden by combination becomes lawful again, as with two sisters, as soon as the one under him is separated by khul' (divorce at the wife's initiative for compensation), by judicial annulment (bat'), or by the end of the waiting period of a revocable divorce ('Sharh Mukhtasar Khalil', 3/210-211; 'ash-Sharh al-kabir' with 'Hashiyat ad-Dasuqi', 3/64-65; 'Tahbir al-Mukhtasar', 2/608-609; 'Hashiyat as-Sawi', 5/38-39). The Malikis apply to the second contract the regime of proof and oath described above.
Second type: he contracts with one and then with the other; this admits two situations. First situation: he contracts with one and then with the second, knowing that she is the second. There is no disagreement that the marriage of the first is established and valid, since there is no combination in it; the marriage of the second is void, for the contract settled on the first before the combination: the second becomes forbidden as of the contract on the first, and no valid contract on her as long as the first has not separated from him and her marriage has not ended.
Second situation: he marries them in two contracts without knowing which is first. Here the jurists differed. The Hanafis: he need not ascertain it, but he is separated from both; one of the marriages is certainly void, with no way to identify it (absence of priority) nor to restrict amid ignorance; separation is therefore imposed, and he owes half of the dower (half of the named dower), for it was due to the first and priority has vanished: it is paid to both ('al-Jawharah an-Nayyirah', 3/274).
The Shafi'is: if he doubts which of them he married first, this is of two kinds. One: the doubt arises after a prior certainty, and both marriages are suspended, one being a wife of unknown identity and the other unrelated; both are forbidden to him and to other husbands until her situation is clarified. If he expressly divorces one of them, she becomes lawful to others, but remains forbidden to him in all cases; the other remains in prohibition: if he renews the contract upon her, she becomes lawful to him. The second kind: the doubt accompanies the initial contract without prior certainty, and the marriage is void, not suspended, since there is no clarification to await. Does its nullity require annulment by the judge or not? Two views: yes, annulment is required and the doubt and confusion are void, for what is not distinguished between permission and prohibition falls under the ruling of prohibition; or: it is not dissolved except by the ruling of a judge, for knowledge encompasses the fact that a wife is among them, and not knowing which does not justify dissolving her marriage until the one with authority over it, the judge, takes it up.
Al-Mawardi: if a man contracts an invalid marriage with a woman and then marries her sister, this is of two kinds. One: he knows the corruption of the first contract, and the marriage of the second is lawful, whether or not he knew at the time of the contract that she was the sister of the first. The second kind: he does not know of the corruption of the first marriage until he has contracted with the second; this is of two kinds: if he did not know at the time of contracting with the second that she is the sister of the first, her marriage is lawful, for his contracting was not accompanied by a prohibition; if he knew at the time of contracting with the second that she is the sister of the first, without knowing the corruption of the first marriage until he contracted with the second, the marriage of the second is void: he undertook a manifestly forbidden marriage, so the ruling of prohibition applies to the corruption. And Allah knows best ('al-Hawi al-Kabir', 9/205-206).
The Hanbalis: if he marries the two forbidden by combination in two contracts without knowing which is the first, he is separated from both. Ahmad, regarding a man who married two sisters not knowing which he married first: 'We separate him from both', for one of them is forbidden to him with a void marriage without our knowing the lawful one: they became doubtful to him; the marriage of one is valid, but her separation from him is only certain by divorcing both or by dissolving both marriages, so it becomes obligatory, like the case of two guardians giving in marriage without the first contract being identified. If he prefers to separate one and then renew the contract of the other and keep her, there is no harm, whether he does so by drawing lots or not. This falls into three cases:
As for the dower: if he has not consummated with either, one of the two has half of the dower; we do not know which is entitled to it: they agree upon it between themselves; otherwise lots are drawn between them, and the one whose lot comes out receives it with her oath. Abu Bakr said: my choice is that the dower falls away if he is compelled to divorce before consummation; if he has consummated with one, lots are drawn between them: if the lot falls on the one he did not consummate with, she has half of the dower, and the one consummated with has the dower of equivalence for what he made lawful of her; if the lot falls on the one consummated with, the other has nothing and the one consummated with has the whole of the named dower; if he has consummated with both, one has the named dower and the other the dower of equivalence, and lots are drawn between them if we say that what is due in the corrupt marriage is the dower of equivalence; if we say that the named dower is due in it, it becomes due here to each of the two.