Is it permissible for my son to marry his maternal cousin if he was nursed by her mother along with her elder brother?

Your son is not permitted to marry his maternal cousin (daughter of his maternal aunt) if he was suckled by her mother with five full, satisfying sucklings, in which case she would be his foster sister. However, if the suckling was less than five sucklings, then he is permitted to marry her according to the Hanbalis and Shafi'is. The Malikis and Hanafis, on the other hand, hold that the prohibition is established by the milk reaching the infant's stomach before the age of two years, even if it is a small amount.