(Lydia being the leader and Kitty the follower) later in the book? At least somehow, I've gotten that idea, too. I also think P&P2 depicted the relationship in that manner as well.

Lydia definitely comes off as "sillier", IMHO, that Kitty. I've always had the feeling that Kitty has SOME sense, but fun-loving and self-confident Lydia leads her farther astray than she might normally have gone on her own. What I find interesting about this is the fact that Kitty is OLDER than Lydia.