The Ruling on Marrying the Daughters of the Sister's Co-Spouse
Question :
My sister is the wife of a man who has another wife and each of them breast-fed the other's sons. The first wife has daughters that are grown-up, and they were not breast- fed by my sister, so is it permissible for me to marry one of them or not?
Answer:
If the matter is as you have mentioned in the question, then there is no sin upon you in marrying one of the daughters of your sister's co-spouse, as long as your sister did not breast-feed them, and there is no family tie, or other tie through fostering which would make them forbidden to you.
And Allah is the Granter of success.
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 5 Page 228