A woman married a man and gave birth to a boy. After the death of her first husband, she married another person and (Part No. 18; Page No. 198) gave birth to a boy. Is it permissible for the son of the second husband to marry the daughter of the first husband's son?
May Allah reward you best!
It is not permissible for a brother to marry his brother's daughter; because she is one of the Mahrams (unmarriageable relatives) mentioned in the saying of Allah (Exalted be He) about women who are not allowed for marriage:
Forbidden to you (for marriage) are: your mothers, your daughters
to His saying:
...your brother’s daughters, your sister’s daughters
(Surah Al-Nisa'). May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.