Is the cousin considered a brother to his aunt's daughters through fosterage simply because his mother breastfed their brother?

"Suckling establishes the same prohibitions as lineage," as the Prophet, peace and blessings be upon him, said: "Suckling prohibits what lineage prohibits." Thus, the suckled child becomes a son to the wet nurse and her husband, the owner of the milk. This prohibition is specific to the suckled child himself, not extending to his brothers and sisters. Consequently, your maternal aunt's son does not become a brother to you or your sisters through suckling merely because his mother suckled your brother.