Do the brothers of a foster sister (who is also a paternal cousin's daughter) become brothers to the questioner, and do the daughters of the paternal cousin's second wife become his sisters?

Your sister's brothers through fosterage are not your brothers, unless they shared in suckling the milk with you. As for your brothers, they are brothers to your sister through fosterage because of her sharing with them in suckling the milk of your mother. The foster-brother is one who was suckled by the same woman who suckled you, and this is by the consensus of scholars.