Yeah. Sadly, This can only end badly or horrible.
But I think it's going to get worse before it gets better. The boy is apparently already trying to implicate the girl in the murders, and if he really is a sociopath he will be cooking up a scheme to spread the blame around as much as possible.
Problem is, he's the original untrustworthy witness. And for that matter, so is she. And the computer postings aren't much better, because of the Teenage Drama Queen syndrome . . . going to take some good police work to figure this one out.
But if I were one of her siblings, I don't know if I could ever speak to her again.