Actually, I can totally see it...assuming she's talking about the boy in the new Peter Pan flick, and not the girls who've played Pan in the past, or Robin Williams. Jack is the name for impish, not-gonna-grow-up males. Think of Jack Nicholson, or all the pirates of history and film named Jack. It just has a roguish air to it. Which is not necessarily a compliment to your brother, but it's not an insult, either.