Does the questioner become a sister to her male cousin through رضاعة (nursing) if her older brother nursed with her older male cousin, and does her younger brother become a brother to her younger female cousin?

If your maternal aunt's son was suckled by your mother, then he is your brother and your siblings' brother through fosterage, regardless of who was suckled before or after. However, if it was your brother who was suckled by your maternal aunt, then your maternal aunt's son is not a forbidden (mahram) relative to you or your sisters. The fosterage that makes one a forbidden (mahram) relative requires five full, satisfying sucklings.