Is it permissible for a man's son from another woman to marry the man's wife's daughter from another man, or vice versa?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
The marriage between a man's son from a non-wife and the wife's daughter from another man, or between the man's daughter and the wife's son, is permissible according to the majority of jurists. This permissibility is based on the general scope of the relevant Quranic verse and the absence of any lineage (*nasab*) or legal cause (*sabab*) that necessitates prohibition. The fact that one is the brother of the other's sister is not established in the Sacred Law as a cause for prohibition, thus the default ruling of permissibility applies based on the general text of the Quran.
Supporting text
It has been narrated from Taus that such a marriage is disliked (*makruh*) if the child in question was born to the woman after the husband had sexual intercourse with her.