{spoiler alert}

Not a mystery at all, and honestly a pretty boring book by BSC standards. One of the very few BSC books I've read where I wasn't dying to finish it to find out what happens.
Claudia's "perfect" sister, Janine, is acting really strange... borrowing Claud's clothes without asking, wearing makeup, lying to their parents and sneaking around.... it is OBVIOUS to anyone with sense that Janine has a boyfriend and is afraid her parents won't approve, and maybe she's even ashamed/guilty for having a boyfriend because that just isn't like her. Anyway, it's totally obvious that that's what is going on, but Claudia and the others are completely oblivious to that fact. Even when they see Janine get into a car with a guy, they don't put 2 and 2 together! ( )