The Husband is the Owner of the Milk

Question :

A child was brought up in his paternal uncle's house and he was breast-fed by his uncle's first wife. Then after a while, his uncle married another woman and she bore him a daughter. Is it permissible for this child (when he grows up) to marry his uncle's daughter born to him by the second wife from whom he was not breast-fed?

Answer:

If the aforementioned child was breast-fed by his paternal uncle's wife five times or more during his first two years. then he becomes the son of his uncle through breast-feeding, and all of his uncle's children. male and female from all of his wives are brothers and sisters to him through breast-feeding. Therefore, you should know that it is forbidden for the aforementioned child to marry the aforementioned daughter, since she is his sister from his father through breast-feeding-if the situation is as mentioned. For Allah, Most Glorified has said in His Clear Book, when He mentioned those women whom it is forbidden to marry:
"your foster mothers who suckled you, your foster milk suckling sisters".

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

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 132-133