Is it permissible for a woman to breastfeed her brother or her husband’s brother if he is an infant, and what are the implications of that? Does the husband’s brother become like her son? Are there individuals whom a woman is forbidden from breastfeeding?

It is permissible for a woman to breastfeed her brother or her husband's brother, and this does not affect her marriage. The suckling becomes her son through breastfeeding, and mahramiyyah (prohibition of marriage) is established because of it. The Prophet, peace and blessings be upon him, said: "What is forbidden by lineage is forbidden by breastfeeding." There is no one whom it is forbidden to breastfeed.