The daughter of one's brother through breastfeeding is forbidden in marriage Sahih Muslim 1448
Translation
Umm Salama (Allah be pleased with her), the wife of Allah's Apostle (ﷺ), said
It was said to the Messenger of Allah (ﷺ): Is not the daughter of Hamza a suitable match for you? Or it was said: Why don't you propose to marry the daughter of Hamza, the son of Abd al-Muttalib? Thereupon he said: Hamza is my brother by reason of fosterage.