Sh. Ibn al-ʿUthaymīn رَحِمَهُ اللهُ was asked about a girl who wants to marry the son of her paternal uncle. However, her brothers were breastfed by the mother of this man she wants to marry. Is she permitted for him in marriage or is she forbidden for him?

Sh. Ibn al-ʿUthaymīn رَحِمَهُ اللهُ said:

“It is clear from this question that the man who wants this woman was not breastfed by her mum, and that she was not breastfed by his mum. Accordingly, there is no brotherhood between her and the man, and so, it is lawful for him to marry her. As for his brothers who were breastfed by her mother, she is not lawful for them because she is their sister.”

Source: Fatāwá Nūr ʿalá al-Darb (10: 528, no. 5484) of Ibn al-ʿUthaymīn.