Nikah with One’s Son-in-Law
Question
Zaynab married her daughter to Khalid, and a few days later Zaynab became infatuated with Khalid, that is, her son-in-law; she had him give her daughter a talaq and then married him herself. Will this nikah of hers be valid or not?
Answer
Nikah between a son-in-law and his mother-in-law is haram. [ar]ويحرم أم امرأته مطلقا دخل أو لا إن كان العقد صحيحا[/ar] “And the mother of his wife is forbidden unconditionally, whether he has consummated the marriage or not, if the contract was valid” (“al-Durr al-Muntaqa”, p. 323).
Source: Fatawa Mahmudiyya
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