That Which is Prohibited by Kinship is Prohibited by Breast-feeding
Question :
I wish to marry a certain young woman, but there is a problem on which I want to know the ruling of the Islamic Law. I was breast-fed with the son of a certain family, and this girl was also breast-fed with a daughter of this family; that is, she was breast-fed along with the sister of the brother who was breast-fed with him. Bearing in mind that I was not breast-fed with any of the sisters of this girl, and she was not breast-fed by my mother. Is it permissible for me to marry her?
Answer:
If you were breast-fed by a woman (Zainab, for example) and she had also breast-fed another boy or another girl, she would be a sister to you, whether she was before you or after you if the breast-feeding was full and complete, being five or more feedings during the first two years.
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 126-127