My foster brother has a brother only from his mother's side. Is he also my brother?

If a person breastfeeds from a woman, he becomes her son through suckling, and also the son of her husband, the owner of the milk. This prohibition applies specifically to the suckled person himself, not to his brothers and sisters.

So, if your brotherhood/sisterhood with someone is due to you suckling from his mother, then you are a sister to all her children, whether they are her biological children or her suckling children. However, if your brotherhood/sisterhood with him is because he suckled from your mother, or because both of you suckled from an unfamiliar woman, then you do not become a prohibited relative to his brother.

It is a condition that the suckling be five full sucklings for the prohibition to be established.