I'm going to defend the husband here, just because we don't know him and I feel like he's already been judged and condemned!
I went through a similar, but different, situation last year. My OH's eldest son (from previous marriage) wanted to move south, find a job and flat in London (aka be closer to his girlfriend). I leapt at the opportunity and said he should stay with us until he finds a job. It'll be easier to get a flat once he has a job. Thing is... he got comfortable. In fact, he got so comfortable, that if we disappeared for a weekend to visit my OH's dad (up north), we'd get back late on a Sunday to find we had a huge piling of washing up to do. Same every evening. Before we could cook dinner, we'd be cleaning his dishes from the day. My OH couldn't bring himself to say anything, not because he didn't care, but because he's not that sort of person. He's very anti-conflict and would rather put up with things that confront them. Even his own dad was saying to kick him out, he's old enough to look after himself!
Anyway, back to the point, maybe the OP's husband is similar? Perhaps he can't stand up to his sister, who sounds lie she wouldn't back down from a fight. I'd be afraid to leave my husband in that sort of company (who knows what viles things she could fill his mind with) and with my precious few possessions.
I wouldn't know what to do, but sat here, comfortably, I can come up with some plots. Change the locks while they're out, leave their stuff - under cover - on the drive and have the police on speed dial for when they come back? She obviously has other family members. As someone else said, maybe it's their turn to take on the burden. You've done your fair share.