Is my sister's brother through رضاعة (breastfeeding) considered my brother?

Your sister's brother by suckling, if he suckled from your mother, then he is your brother by suckling; because he becomes a son to your mother and a brother to her children. However, if your sister is the one who suckled from his mother, then she is a sister to the children of that mother by suckling, and you are not a brother to him.

The suckling infant becomes a child to the wet nurse and to her husband, excluding the rest of his brothers, sisters, and ancestors. As for his descendants, they are prohibited (for marriage) to the wet nurse and her relatives, just like the suckling infant himself.