Is it permissible to marry the daughter of a woman who was nursed by the questioner's mother and paternal aunt, knowing that the questioner's brother is married to her sister?

If your mother breastfed your female cousin five times during the first two years of her life, she becomes your sister through suckling, and it is not permissible for you to marry her daughters, because they are the daughters of your suckling sister. This is based on the Prophet's (peace be upon him) saying: "What is forbidden by lineage is also forbidden by suckling." And the daughters of a sister are forbidden (for marriage). As for your female cousin's sisters who were not breastfed by your mother, the prohibition does not extend to them. It is permissible for your brother to marry them if there is no other reason for prohibition.