Is it permissible for my son to marry the daughter of my second wife?
All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
This marriage is lawful, because there is no impediment to it. Ibn Qudaamah said, “According to opinion of the majority of the Islamic jurists, it is permissible for one to marry the daughter of his stepmother from a man other than his father and vice versa.”
Allaah Knows best.
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