Her MIl is being awful, but I’m guessing it’s not new that she’s behaving inappropriately. Considering all DD is struggling with, it’s SILs role to handle supporting his brother and his wife; not his wife’s. It’s not MILs place, and if SIL was my friend I’d tell him he should tell his mom to put a cork in it and not meddle (and please stop passing on nastygrams to his wife!). Support your Dd and take care of yourself as best you can.
What gives you a warm feeling? 🥰