Today, my sister admitted to having sex with her current boyfriend. On my bed. Again. I even have a spare bed in my room and yet they still go for mine. FML
Woah okay was not expecting this to get published. I'm OP. Okay so first off my sister and I are really close and always have been so she's been forgiven and we can joke about it now. And secondly, we were both pretty amused by the suggestions of ways to get her back, I think my favorite is "punch her in the head". And I would totally buy a lock if I could, by the way.
Put cayenne pepper or anything like it in the one they keep having sex on, and sleep in the other one. It SHOULD cause some burning, itching, maybe some sores and a yeast infection. Worst case scenario is it gets in her eyes and even then they should be fine afterwards.
You could also put raw rice in the bed- I bet it's hard to stay in the mood when it feels like your laying on tiny pebbles. I'd say thumb tacks, but that's a little too dangerous. Or even just some good old itching powder if you can find it. Fake roaches might kill the mood.