2645 - Narrated Ibn ‘Abbaas (may Allah be pleased with him) that the Prophet ﷺsaid about Hamzah's daughter, "It is unlawful for me to marry her, as foster relations are treated like blood relations (in marital affairs). She is the daughter of my foster brother."
.

Commentary : Islam decreed that breastfeeding should create a bond (i.e., milk kinship) similar to blood relationships and forbids exactly the same which is forbidden due to blood kinship. Breastfeeding establishes milk kinship, and accordingly the prohibition of marriage is established just like the case with blood kinship. Hence, it is forbidden for a man to marry his sisters through nursing or his nurse-mother, or his maternal or paternal aunts through nursing, and so on.
In this hadeeth, ‘Abdullah ibn ‘Abbaas (may Allah be pleased with them) stated that when the Messenger of Allah ﷺ was offered to marry the daughter of Hamzah ibn ‘Abd Al-Muttalib (may Allah be pleased with them), he ﷺ stated that she was unlawful to him, being the daughter of his milk-brother. Thuwaybah, the freed-slave of Aboo Lahab nursed both Hamzah (may Allah be pleased with him) and the Prophet ﷺ, and Hamzah was two years older than the Messenger of Allah ﷺ.
He ﷺ said: “… foster relations are treated like blood relations (in marital affairs),” meaning that milk kinship prohibits that which blood kinship does. Accordingly, since it is unlawful for a man to marry his niece (his blood brother’s daughter), it is similarly unlawful for him to marry his milk-brother’s daughter.
.