See, here's the thing.
Based on what you've told us, I would think that there is some chance that she will take legal action to try and take the kids.
IF she attempts to do so, she might lie and lie big.
What you should do is be honest in that case, and I do believe it will help if you keep track of "everything" that she tells you. Be it in email, mail, over the phone et cetera.
Get a folder going.
Document the madness!
Just in case you have to prove just how much madness you've had to deal with at her hands, the sort of madness your kids would have to deal with if she'd have her way.
So that you're perfectly equipped to show, clear as day, that they are better off with you.
Also, I'd make sure they aren't ever left unwatched.
You never know just how far extremists will go.
I'd hate to read a quick message on the forum where you're telling us that you'll be afk a while due to your mum kidnapping her grandchildren.