It becomes forbidden by sucling those relationships....

It becomes forbidden by suckling those relationships that are forbidden by kinship Q. I want to marry a particular girl, but there is a problem and I want to know the ruling of the Sharee’ah concerning it: I suckled alongside a son of a particular family and this girl whom I want to marry suckled alongside a daughter of this family as well. That is she suckled alongside the sister of the brother who I suckled along with. Bearing in mind that I did not suckle alongside any of this girl’s sisters and she did not suckle with my mother, is it permissible for me to marry her?

A. If you suckled from a woman and this girl suckled from her as well, alongside another boy or girl she is your sister in suckling, even if she suckled before or after you, if the feedings were complete, that is five or more and you were both younger than two years old when suckling took place.

Sheikh Ibn Baaz [Fataawa Islaamiyyah: 3/329]