get a camera, hook it up to your sister's room and catch her in the act, bring all your evidence to the police.

or do brute force, barge into the room with a weapon and whoop her ass dead

my idea is that some kind of demon possessed somebody and the original person stole their faces to turn them into a form of void like the first person, and the demon's goal is to possess everybody by removing their faces and replacing them with the void

now that last sentence is a little bit suspicious mate

would be funny if a short skeleton with a blue zip up hoodie and gym shorts was beating you up in the middle of the night