the guy

Is she even really at her sister place?

I mean there a some big red flags here.

Can you verify if she is, I mean you have to start really looking at whats really going on, by planting a VAR (Voice ativated Recorder) in her car, go to BestBuy and pick one up with some velcro tape, tape it under her seat.

See all the help in the world won't matter until the OM .....or OB is out of the picture.

Even if it not the kid it maybe be a new kid.

On a side note, it sound like your SIL got caught up with a student and when she tried breaking it off the stundent blackmailed her...hence the rape acusation