What is the ruling on my brother marrying his maternal aunt's daughter, who breastfed with his half-sister from his mother, with different fathers, and the mother is ignorant of the number of breastfeedings, and is my brother considered an uncle to the bride?

If your mother breastfed your maternal aunt, then your aunt becomes your mother's daughter through suckling (rada'a), and your mother's children become siblings to your aunt through suckling. Consequently, it is impermissible for your brother to marry your aunt's daughter. The prohibitive suckling requires five separate sucklings, in which the suckling infant draws milk into its stomach before reaching two years of age. This is the view of the Shafi'i and Hanbali schools. If there is doubt about the number of sucklings, the prohibition is not established. We advise your brother to separate from his maternal aunt's daughter and divorce her as a precautionary measure, because the Hanafis and Malikis state that the prohibition is established with a single suckling.