Zina with the Wife’s Sister and Prohibition by Affinity

Question

If someone committed zina with his sister-in-law (his wife’s sister), and did so by mistake, in a state in which it seemed to him that she was his wife, and afterwards it became known that she was his sister-in-law — tell us, did his wife remain in his nikah or leave the nikah? Please write the proof. And if he knew that she was not his wife but his sister-in-law, what is the ruling?

Answer

His nikah remains; it has not ended. However, if he had intercourse (wat’) with this sister-in-law thinking her to be his wife, then this is intercourse by mistake (wat’ bi’l-shubha). In this situation he should keep away from his wife until the sister-in-law has a menstrual period (hayd).

Source: Fatawa Mahmudiyya

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