The Breast-feeding Which Causes Prohibition

Question :

Breast-feeding prohibits marriage between the two children who were breast-fed, but does it prohibit marriage between all of the brothers and sisters on both sides? We request clarification, may Allah reward you with goodness.

Answer:

If a person is breast-fed in a manner according to the Islamic Law, by which prohibition occurs and that is five breast- feedings or more, when the breast-feeding takes place during the first two years then that results in it being prohibited to marry the breast-feeding woman and her mothers (i.e. mother, grand- mother etc.), her sisters, her paternal aunts, her maternal aunts, her daughters, her sons' and her daughters' daughters and their descendants whether they are from the husband or other husbands, according to the words of the Prophet ﷺ:
"That which is forbidden due to kinship is forbidden due to breast- feeding."

But it is not forbidden for his brothers who were not breast-fed by the woman who breast-fed him to marry the daughters of the breast-feeding woman. This is because she is not a mother to them, since she did not breast-feed them, only their brother.

Nor is it forbidden for her sons to marry the sisters of the one who was breast-fed by her. This is because they are not her daughters, nor are they sisters to her sons, since there was no breast-feeding. And all that we have mentioned is clear from the words of the Prophet:
"That which is forbidden due to kinship is forbidden due to breast-feeding."

And Allah is the Granter of success.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Page 146