Sounds like she gets off on the fact that she's having sex on your bed- or that her BF does. Which means your sister either wants to be you, has sexual feelings towards you, or the BF is pretending your sister is you. But that's just what I get from the fact you've had to deal with this so often. 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.

"No more monkeys jumping on the bed"