I hope you give your DH lots of hugs and support and tell him please, 1,000,000 times, that he’s a good son and he’s doing the very best thing.
And when SIL is finally in jail, I think a lot of people will be doing the Snoopy dance. I think right now you already have enough to lock her away for good. I really hope she gets a lawyer involved because then you can bring all this crap to light, get her put away, get your MIL in better quarters, and leave all this behind you. Lots of hugs from here; hang on as best you can. I think this ride is going to be over soon.