If a woman breastfeeds a child, he becomes her son, and all her children become his siblings

Question:
I have a maternal cousin, and my mother breastfed her along with my younger brother. Is it permissible for me to marry her or not?

Answer:
If your mother breastfed you five times within the first two years, then she becomes your sister, even if she was breastfeeding you along with your younger brother. This is because if a woman breastfeeds a child five times within the first two years, that child becomes her son, and the son of her husband to whom the milk is attributed. All of their children become siblings to this breastfed child, whether they were born before him, at the same time as him, or after him; they all become his siblings. Yes.