No problem. She’s guilty of breaking and entering. And destruction of private property. If your husband sides with her then I’d take a close look at your marriage. Nobody can take advantage of you without your permission.

So that's when you call the police and have her arrested. There is no reason whatsoever to allow her to break into your home and get away with it.

Make your husband buy you a new mattress and replace your favorite sheets. If he wants to share your house with his filthy sister, he should pay to keep your standard of living the same. Unless he's really rich, this will cause him enough pain to divert her to the nearest sty when she gets sleepy.

Who has a favourite set of sheets?

three-some?

Are you the only one authorised to hand out keys? Obviously your husband gave her a key. It’s his house too isn’t it? I think the two of you need to sit down and have a long discussion. I sense there’s tension in your marriage.

Burn the mattress an have a long talk with your husband. He should have told you that his sister had a key an how she got it. Or don’t sleep in the same bed with him until he fixes the entire situation