That Which is Forbidden Due to Kinship is Forbidden Due to Breast-feeding
Question :
My son was breast-fed by his grandmother on his mother's side and so of course, he became a brother due to breast-feeding for his maternal aunts and uncles; so is it permissible for him to marry his maternal aunts' daughters or his maternal uncles' daughters?
Answer:
If the aforementioned child was breast-fed by his grand- mother on his mother's side five times or more while he was in the first two years of life, then because of that he becomes a brother to his maternal uncles and aunts, and a paternal uncle to the children of his maternal uncles and aunts. So it is not permissible for him to marry his maternal uncles' daughters, because he has become a paternal uncle to them through breast- feeding. Nor may he marry his maternal aunts' daughters, because he has become a maternal uncle to them, and their descendants, through breast-feeding.
And Allah is the Granter of success.
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Page 117