Even knowing that, my only response would be, "You have 24 hours to get the hell out of my house." And I'd call the police to make sure she left. I might let let the boyfriend stay, but he'd have to choose between her and the house, and he'd have to sign a lease and pay a nominal rent to make it official.
Well, the thing is, the boyfriend can't stay there without sister. He's right in the middle of a kerfuffle with Social Security - his job was paying him 50 cents an hour too much for him to keep his disability benefits. And now that he's incapable of working any more (he has multiple sclerosis and it's gone downhill BAD) they say he's no longer qualified to get the benefits back AND they want him to pay back everything he GOT from them before, to the tune of several thousand dollars. He's not capable of living on his own any more, much less being able to afford it.
I spoke with my mom earlier and she's feeling much better, she's past "upset" into "full-on rage" and she did talk to sister again today. No more talk of throwing things out, so I think it really was just an itch-with-a-B moment. We'll see what happens. I'll definitely make a whole thread if she pulls any more shenanigans.