The Sisters of Your Brother Through Breast-feeding Are Not Forbidden to You
Question :
I have a cousin who breast-fed with me when I was small child for a period of ten days or more. He has two full. sisters younger than he; is it permissible for me to marry the youngest of them?
Answer:
If the matter is as stated, then it is permissible for you to marry one of the sisters of your aforementioned cousin, as long as there is no breast-feeding between you and those from whom you wish to marry which would prohibit it. The fact that he was suckled by your mother does not effect your marriage to any of his sisters, whether that suckling was a little or a lot.
And Allah is the Granter of success. May peace and blessings be upon our Prophet, Muhammad and upon his family and Companions.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 6 Page 157