Q: A man called Sa`id married two women; the first bore him a son called `Abdullah and the second bore him a daughter called Fatimah. `Abdullah got married and had a daughter called Salma, who later got married and had a son called Ahmad.
Is it permissible
for Ahmad to marry Fatimah?
It is impermissible for Ahmad to marry Fatimah, because she is his mother's paternal aunt, which means that she is considered as his aunt. All his parents' and grandparents' paternal and maternal aunts are considered aunts for him. Allah (Glorified and Exalted be He) says,
Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters
Muslim scholars have unanimously agreed on this. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.