Your sister will probably already figure you for a geek/otaku and a pervert so it shouldn't be that big of an issue. I mean she is 18? If she isn't staying in your room then keep it locked or if you are that nervous store some of the more lewd shit where any pervert would: under the bed, in the closet, in boxes, or under the floor boards.
Also figured this video to be appropriate.
There's actually a videogame called Lose/Lose that destroys the data on the harddrive. This means everything. Literally. The OS is not safe from this. Those encrypted files are not safe. The game itself is not safe. Everything goes.
Well you could just be a huge asshole and pull the "its my house I can have tits on my wall" card because if shes stayign at your place she should be thankful to have a place to stay even if it smells of self-satisfaction :P