I think Cassie is actually a pretty good representation of an only child, if you're only focusing on the negative stereotypes: spoiled enough to think her way is the only one that has any merit, kind of a self-centered worldview (I mean not in her actual opinions, just, like I said, the fact she was normally unwilling to compromise her "morals" and opinions. Compromise is the first thing you learn when you get a sibling.) Jake...I don't know, I agree with you, but I've heard that if two children are close enough in age their "roles," ie "Big brother" and "little brother" may be reversed. But Tom did kind of seem like a "big brother" before he got infested, what with Jake wanting to play basketball just because his older brother did...um, idk. I did feel like Rachel was a big sister, especially in the early books, and iawtc about Marco and Tobias, it's hard to say because Tobias never really had a "family" in the traditional sense of the word and Marco had to accept very adult responsibilities very early since his dad kind of gave up after Eva "died." Jake's probably the one I have the most problem with, like you said.