It is not valid for him to choose one whose marriage is not valid. The mother is singled out for the invalidity of her marriage because she becomes forbidden upon the mere act of contracting with her daughter forever, so it is not possible to choose her. The daughter, however, does not become forbidden by her mother before consummation, so the marriage to her is confirmed, unlike the case of two sisters.

Section Two: If he has consummated with both of them, they both become forbidden to him forever: the mother because she is the mother of his wife, and the daughter because she is his stepdaughter from his wife with whom he has consummated. Ibn al-Mundhir said: All those whom we have memorized from among the scholars have reached a consensus on this. This is the opinion of Al-Hasan, Umar ibn Abd al-Aziz, Qatada, Malik, the people of the Hijaz, Al-Thawri, the people of Iraq, Al-Shafi'i, and those who followed them. If he consummated only with the mother, it is the same; because the daughter becomes his stepdaughter, with whom he has consummated by way of her mother, and the mother was forbidden by the mere act of contracting with her daughter. If he consummated only with the daughter, her marriage is established, and the marriage of her mother is void, just as if he had not consummated with either of them. If only one of them converted along with him, the ruling is the same as if they had both converted with him. If the one who converted is the mother, she is forbidden to him in any case. If it is the daughter, and he has not consummated with her mother, her marriage is established; but if he has consummated with her mother, she is forbidden forever. If he converts and he has two slave women, one of whom is the mother of the other, and he has had intercourse with both of them, they both become forbidden to him forever. If he has had intercourse with only one of them, the other becomes forbidden forever; the one who was had intercourse with is forbidden [even if he had not had intercourse with either of them, he may have intercourse with whichever of them he wishes; then, when he has had intercourse with one, the other becomes forbidden forever]. And Allah knows best.