Well, mother in law will SOON find out what that girl is all about, and they won't believe her in time. People like her don't last long. Those lies get found out sooner or later. So i wouldn't worry about it.

If mother in law contacts you again, just tell her to keep on eye on that girl. Tell her that she'll "soon find out" that YOU are right and let it go at that. Then maybe she'll keep her eyes open. Maybe she won't too, but she can't say you didn't warn her! Lots of luck, hon. Hugs, Lee