Who lets a twelve or thirteen year old babysit an eleven year old?
It doesn't seem like that much of a stretch, that's around the age most kids are able to be given some responsibility, and it wouldn't so much be baby-sitting as much as having an older kid keeping an eye on them.
Happened to this troper when he was eleven. Granted, that was the last time I had a babysitter but it does happen.
It's a common tactic to prevent the child from feeling like they're being babysat. Get someone around who's closer to their own age and they'll feel a little less immature.
I started watching my 4 years younger brother in late elementary school. We'd ride the bus home and I'd make sure he stay out of trouble until my mom got home.
It's also been shown that Susie is very mature for her age, so they probably trust her more.
What is Chuckie's problem with Tommy dating his sister? He knows he's a perfectly nice guy.
Yeah, but the three of them grew up together. As someone pointed out on the YMMV tab, the Westermarck Effect would be in full force by this point.