The Breast-feeding of Your Brother With Your Wife's Sister Does Not Affect Your Marriage
Question :
A man's eldest brother was breast-fed along with his wife's sister by her mother. Is there any effect from this breast-feeding on her sister remaining under his authority?
Answer:
The breast-feeding of the questioner's eldest brother along with his wife's sister by her mother does not affect the continued validity of the questioner's marriage to his wife. This is because the fact that his wife is a sister to his brother through breast-feeding does not make her forbidden to him; it only makes her forbidden to his brother if he was breast-fed five times or more, and it took place during the first two years of his life. And Allah is the Granter of success. May peace and blessings be upon our Prophet ﷺ, Muhammad and upon his family and Companions.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 6 Page 136