u/FangArtistry

As the title says. I 18m still live at home (currently in college) with my sister. 22f. To add a bit of backstory about myself unfortunately I was raped as a child by a family member repeatedly and I've been sexually assaulted twice by different people in my later teens. Anyway onto what happened. So I sleep naked and I make sure everyone is out of my room before I undress to go to sleep. So my sister will lay in my bed and kinda just laze around until I have to kick her out of my room, (not my choice, she doesnt respect boundaries.) So I said goodnight, she left my room (I heard the door close.) So I undress and then go to pick up my charger so I am bent over naked, and I feel a slap on my butt. She came back into my room and slapped my ass. I am so unsure about what to do with this situation. I have told everyone in my family do not touch me if im accidentally naked, which does not happen because I always wear clothes around them. I feel really disgusted and I had a really awful panic attack, in which my told my mom about the situation and she downplayed it i feel. Is this sexual assault?