The Prophet (ﷺ) said about Hamza's daughter, "I am not legally permitted to marry her, as foster relations are treated like blood relations (in marital affairs). She is the daughter of my foster brother."
Sahih al-Bukhari Book 52, Hadith 9
← Previous Hadith Aflah asked the permission to visit me but I did not allow him. He said, "Do you veil yourself before me although I am your uncle?" `Aisha said, "How is that?" Aflah replied, "You were suckled by my brother's wife with my brother's milk." I asked Allah's Messenger (ﷺ) about it, and he said, "Aflah is right, so permit him to visit you."

Sahih al-Bukhari Book 52, Hadith 8 Next Hadith → That `Aisha the wife of the Prophet (ﷺ) told her uncle that once, while the Prophet (ﷺ) was in her house, she heard a man asking Hafsa's permission to enter her house. `Aisha said, "I said, 'O Allah's Messenger (ﷺ)! I think the man is Hafsa's foster uncle.' " `Aisha added, "O Allah's Messenger (ﷺ)! There is a man asking the permission to enter your house." Allah's Messenger (ﷺ) replied, "I think the man is Hafsa's foster uncle." `Aisha said, "If so-and-so were living (i.e. her foster uncle) would he be allowed to visit me?" Allah's Apostle said, "Yes, he would, as the foster relations are treated like blood relations (in marital affairs).

Sahih al-Bukhari Book 52, Hadith 10