Is it permissible to marry the sister of a brother through fosterage, knowing that I was nursed by her mother?

What is established by lineage is also established by fosterage. Therefore, whoever suckles from a woman becomes a child to her and to her husband, the owner of the milk. If you and your brother's foster-sister both suckled from the same woman, or if the milk you suckled from that woman belonged to the girl's father, then she is considered your foster-sister. However, if she is your brother's sister only from the paternal side, and she did not suckle from her father's milk, and neither of you shared fosterage from any woman, then it is not forbidden for you to marry her. Her being your brother's foster-sister alone does not make her unlawful for you unless there is another reason for the prohibition. It is stipulated that five satisfying sucklings are required for fosterage to constitute a prohibition.