My bfs sister broke my trust


A week before Christmas my bf’s family had a Christmas party and I attended. I felt kind of awkward throughout the night since a lot of the people there weren’t too friendly so I sat in the back room with my bf and his sisters plus one of his sisters boyfriends. Later on in the night they all left to socialize and I stayed back there with my boyfriends autistic sister. She’s 15, nonverbal and she likes to play “tickle” with me and that’s how we bond. She asks me to tickle her and we would usually do that for a bit. So when we were in the back I was on the couch and she was on the recliner and pulled me to tickle her. When I did she told me “sit” and I was like wow I can’t believe she verbalized that!! So I sat in front of her like she wanted and I realized she went quiet, when I turned to check on her her eyes were fluttering so I assumed she might have been having a seizure, when I turned the rest of my body I felt her humping me. I was extremely uncomfortable and felt violated so I jumped up. I was stunned and went to look for my boyfriend but I couldn’t find him so I went back to the couch and sat as far as possible away from her. Then she started getting angry and insisted that I sat with her and I didn’t want to be mean so I just kept making excuses to leave the room. I considered telling my bf’s mom but she is extremely protective of her children and I felt she would blame me. I told my boyfriend about it but he never told her and now I don’t want to be left alone with her which makes me feel like a jerk but I did feel hurt. She has masturbated publicly before and her family would seemingly handle the situation but I’m worried that she would do this to someone else and might get hurt in self defense. What should I do?