Sooner, I'm floored that anyone can behave as your SIL is under these circumstances. More indicators how wise your DD is for getting away. I hope he tires of the drama soon. Meanwhile, will your local PD, who has already witnessed his abusive behavior, do regular checks/drive-bys during the night? (And while I've never shot anything more alive than a clay bird, I did discover years ago that just the sound of racking a shotgun could deter the jerk trying to get through my back door.)
I bought the VIVE Alternating pressure pad to go over my husband's rock-hard hospital bed mattress (from a big online store) and he likes it. To keep it from slipping down, it has "tails" that tuck under the head and foot of the mattress, and then I cover the pad and mattress with a fitted sheet, then a synthetic sheep skin the size of the mattress, then another fitted sheet. And I get him into bed exactly like Jim describes with Darcie. It's important to get his bottom in the trough created by the raised head of the bed and slightly raised foot portion and then I can lower the head & adjust the foot till he's comfortable (always have to have the foot section raised at least some to create that trough). If I get him into that "sweet spot," I don't need to pull him up all night. The pump is quieter than the Trilogy, so it doesn't bother either of us. And the firmness is adjustable--getting that right is important for comfort, and just a matter of trial and error.
Sure do hope things calm down. You have enough on your plate without all the SIL drama.