When Emily first encounters Charlie she ask him incredulously who he is. However, it's been established that, although previously an off screen character, Emily is has been home during breaks from school in the ensuing years and thus must know who Charlie is. In addition it's been established that Charlie has been in the Westin's lives for many years as Harry's wife Libby was the one who enabled Charlie to mooch of the family as she felt sorry for him. Thus Emily would have known who Charlie was as well as having an close, though crazy, relationship with him just as Harry, Barbara, and Carol do.