He was breast-fed by the wife of his maternal uncle; can he or his brother marry any of her daughters?
Question

I was breast-fed by the wife of my maternal uncle, along with one of her daughters, so I became a brother to this daughter through breast-feeding. Is it permissible for me to marry one of her sisters under any circumstances? Is it permissible for my brothers to marry the daughter with whom I was breast-fed, or any of her sisters?
Praise be to Allah.In your case, as you were breastfed by the wife of your maternal uncle, you do not have the right to marry any of them, either the daughters of the wife or the daughters of the husband, even his daughters from another wife, because you have become a son to both of them, if the breast-feeding definitely occurred five times or more before you reached the age of two. In that case you are a son to the woman who breast-fed you and a son to her husband, the “owner of the milk”, and a brother to all of their children, a brother to the children of the wife from this husband or from another husband, and a brother to the children of the man, the “owner of the milk”, from this wife or any other wife. So he became your father, and she became your mother, and you do not have the right to marry any of their children.
As for your brothers, there is nothing wrong with them marrying the daughters of this man if they were not breast-fed [by his wife], because they are not brothers to them. Rather the status of mahram [and prohibition on marriage] applies to you only, because you are the one who was breast-fed by their mother, so you became a brother to them. As for your brothers, there is nothing wrong with them marrying them, if there are no ties through breast-feeding between them and these girls, and the girls were not breast-fed by your mother or another wife of your father or any of your sisters. What is prohibited is for you to marry any of them, because you have become a brother to them; as for your brothers, they are “strangers” [non-mahrams] and are not brothers to them. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).
Fataawa Noor ‘ala ad-Darb (3/1553).