J4J I'm very concerned that there doesn't seem to be an aspect of your life at this difficult time that your H hasn't infiltrated (with the exception of your wonderful friend waterlego)
He's staying with your parents (and who knows what crap he's feeding them while he's there - with his manipulative track record I shudder to think) which has effectively cut them off as a source of support for you, although I'd argue they sort of did that themselves but allowing him in.
And then on the other hand, he also has his own mother becoming ever more increasingly involved. You don't know that he hasn't put her up to spending more time with your DCs (is she going to be reporting back? keeping your H informed on what's happening in your house?)
I would think very carefully before aligning yourself with anyone that seems to have your H's interests at heart rather than yours. When it comes to your MIL, only you can judge the situation, but I would tread with caution.
I would also consider calling your parents and being extremely blunt with them - tell them that you view the fact that they are housing the man who assaulted you as a gross betrayal, and that as long as he is there, you will consider them complicit in your abuse.
It makes me very
on your behalf that your H is getting away with this at your parents. Absolutely disgraceful IMO, you must be very hurt.