I have a brother who has a son and this son has daughters. I would like to marry one of this son's daughters. Is it permissible for me to marry any of them?
If your brother, whom you mentioned, is a full-brother or a half-brother from the side of your father or mother or a foster-brother, it is not permissible for you to marry any of his daughters or his son's daughters and their descendants. Allah (may Allah be Exalted) says:
Forbidden to you (for marriage) are: your mothers, your daughters
to His saying:
...your brother’s daughters, your sister’s daughters, your foster mothers who gave you suck, your foster milk suckling sisters
The Prophet (peace be upon him) said:
Milk relationships are equal to blood relationships in rendering marriage unlawful, i.e. whatever is prohibited due to blood relationships is also prohibited by milk relationships.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.