What is the ruling regarding marriage to a mother and her daughter when they convert simultaneously before consummation?
Chapter on Marriage of Polytheists
Al-Mughni
Book of Marriage
Primary text
The marriage to the mother is nullified, and the marriage to the daughter is established. This is one opinion of Al-Shafi'i and preferred by Al-Muzani. The reasoning is that the proof lies in the verse {And the mothers of your wives} [Quran 4:23], and since she is the mother of his wife, she falls under the general prohibition. Also, she becomes permanently forbidden as the mother of his wife, similar to if he had divorced the daughter while in disbelief. If he had married only the daughter and then divorced her, her mother would be forbidden upon his conversion to Islam, so maintaining her marriage is stronger. The view that the contract is only valid upon choice is incorrect, as the contracts of disbelievers are valid, and their rulings established. The mother is singled out for nullification because her marriage becomes permanently forbidden merely by contracting the marriage to her daughter, preventing choice, whereas the daughter is not forbidden before consummation with the mother, unlike the case of two sisters.
Supporting text
If consummation occurred with both, both are permanently forbidden—the mother because she is the mother of his wife, and the daughter because she is his stepdaughter from a wife with whom he consummated. This is agreed upon by all scholars whose opinions we know. If consummation occurred only with the mother, the same ruling applies because the daughter becomes his stepdaughter from a wife with whom he consummated, and the mother is forbidden merely by the contract with her daughter. If consummation occurred only with the daughter, her marriage is established, and the marriage to her mother is annulled, just as if no consummation took place. If only one converts, the ruling is as if they both converted together: if the convert is the mother, she is forbidden in all cases; if the convert is the daughter and there was no prior consummation with the mother, her marriage is established; but if there was consummation with the mother, she is permanently forbidden. If he possessed two female slaves, one being the mother of the other, and he consummated with both, both are permanently forbidden. If he only consummated with one, the other becomes permanently forbidden, but the one he consummated with is not forbidden. If he did not consummate with either, he may have intercourse with whomever he chooses, and upon doing so, the other becomes permanently forbidden.