Um, you're fucked. Seriously. Let your girl handle it, tell her it would be best if her mom found new living arrangements at her earliest convenience. Her mom ain't exactly an old lady (considering that I'm 45 myself), she should certainly be able to manage to find appropriate accommodations. Other than that, keep your head down and your mouth shut.
See, I've lived with my husband in my mother's home, then we lived in his mother's house, and then he and I and my mother purchased a place together.
The only one that had reasonable equity was the final situation. Can you live with parents/in laws? Yes. But only if you find a place you can all agree on and buy it together and everyone's on the deed and mortgage and you've got agreements put into place ... oh, and insurance, that's where I dropped the ball.
Or they're just so old that they have no choice, they need you, then they keep their opinions to themselves.