Each time my sister's boyfriend comes over, I want to leave.
Is he that bad to be around?
I'm afraid that they would have sex in my room.
Others having sex in your room or even in your whereabouts is surely uncomfortable. To make sure you and your sister and fine, you should have a kind conversation about your concerns and protect your personal space. Set some boundaries.
I guess. As much as I would like to prevent that from happening, I don't like them. I should tell my sister and maybe get a room on my own.
That is your room though so you are right to voice your concerns. If you are bothered by something like that you should definitely let them know.