Shaykh al-Islam Ibn Taymiyyah, may Allah have mercy on him, differed on this matter, holding that it is permissible to be married simultaneously to two foster sisters, or to a woman and her foster paternal aunt, or to a woman and her foster maternal aunt. However, his view is weak, and the truth is more deserving of being followed. The hadith is clear: “Fosterage makes unlawful what blood relationship makes unlawful,” whether the prohibition is due to the relationship itself or to a reason connected to it. Thus, whatever women are forbidden through blood relationship in themselves are also forbidden through fosterage, and whatever is forbidden because of a reason connected to blood relationship is likewise forbidden through fosterage. Accordingly, one may not be married simultaneously to two foster sisters, nor to a woman and her foster paternal aunt, nor to a woman and her foster maternal aunt.

“If she is divorced and her waiting period comes to an end, they become permissible. But if he marries them both in one contract, or simultaneously in two contracts, both contracts are invalid” ...

His statement, “If she is divorced and her waiting period comes to an end, they become permissible,” means that the wife’s sister, her paternal aunt, and her maternal aunt become permissible. However, the author stipulates that the waiting period must first come to an end. The apparent meaning of his words is that, so long as the waiting period remains, they are forbidden, whether it is the waiting period following an irrevocable divorce or otherwise.

But we return to what was mentioned previously: the stronger view is that if it is a major irrevocable separation, there is no problem, since it is impossible for him to be married to them simultaneously. As for a minor irrevocable separation and a revocable divorce, he may not marry the sister of a woman whose waiting period results from a minor irrevocable divorce or a revocable divorce.

It says in al-Rawd (1): “If a man has intercourse with his wife’s sister under a mistaken belief, or commits zina, then his wife becomes forbidden to him until the waiting period of the woman with whom he had intercourse has ended.” That is, if a man were to commit zina with his wife’s sister—may Allah protect us—we would say to him: Your wife is forbidden to you until the waiting period of the woman with whom you committed zina has ended. If it so happened that she became pregnant from this intercourse, then his wife would not become lawful for him until the woman with whom he committed zina gave birth to her child, even if the child remained in her womb for four years! However, we have previously mentioned that the stronger view is that zina