The Breast-feeding of the Paternal Uncle's Daughter With the Brother

Question :

My paternal uncle has a daughter whom I wish to marry, but it has been made known to me that she was breast-fed along with my brother who is younger than the brother who came after me. She was breast-fed more than five times, as was my brother; what is the ruling of the religion in this matter, is she permissible for me or not?

Answer:

If the aforementioned girl was breast-fed by your mother five times or more, during the first two years of her life, then she is a sister to you and to all of your brothers from your father and mother if she was breast-fed when your mother was married to your father.

But if she was married to a man other than your father, then she will be a sister from your mother through breast-feeding to you and to all of her sons from all of her husbands. This is in accordance with the Words of Allah, Most Glorified, Most High in Surat An-Nisa':
"your foster mothers who suckled you, your foster milk suckling sisters."

And, the saying of the Prophet ﷺ:
"That which is forbidden due to kinship is forbidden due to breast- feeding."

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 122-123