Are the second wife of my foster father and her daughters considered unmarriageable to my brothers, and am I unmarriageable to the sons of my foster mother's husband?

Your aunt's husband, from whom you suckled, is considered your father through suckling; for suckling prohibits what birth prohibits. Accordingly, his wives, daughters, and sons become mahrams to whoever suckled from his milk. As for your brothers who did not suckle from your aunt's husband's milk, they are not considered mahrams to his wives or to his daughters.