Is it permissible to marry the sister of one’s brother through breastfeeding?

Question:
May Allah reward you. The questioner says: My brother was breastfed by a girl’s mother, so is it permissible for me to marry her?

Answer:
If your brother was breastfed by the girl’s mother, she becomes his sister through breastfeeding. As for you, you are a stranger to her, so you may marry her. You may marry your brother’s sister through breastfeeding because you are a stranger to her. However, if she was breastfed by your mother, if she was breastfed by your mother along with one of your brothers, then she becomes a sister to all of you. It is not permissible for you to marry her because she was breastfed by your mother; she has become your sister. But if it was your brother who was breastfed by her mother, she becomes a sister to him specifically, and not to his brothers, so you may marry her.