What does Sylvia realize about her brother during their night out at the Blu Night in the novel, The Vacationers?

When Bobby takes Sylvia to the Blu Night, his goal is to blow off steam because he is angry at Carmen. For Bobby, it is a seemingly normal night of alcohol, dancing, and women. For Sylvia, she leaves a bit of her innocence behind as she realizes that her brother is not a very nice guy.