I'm [17f] afraid of my cousin [21m] and his violent and sexual tendencies.

My cousin has been living with us since he was 14. My mom's brother [my uncle] his dad, and his mom pretty much gave up on him and dropped him off one night saying they "couldn't handle him anymore" and I think they eventually gave up custody of him to my mom. He has...issues. Like he will get angry at his video games and smash the controller. He's broken 2 TVs in his room so far. His walls are full of holes and one time we had to take his door off the hinges because he kept kicking holes into it. My mom will just buy him new games, controllers, a new PS4 whatever just to make him happy and not anger him. He is on anger medication but it seems like it doesn't help. He's been in therapy for years for his issues but I don't know if it actually helps him. Every other word out of his mouth is "Motherfcker or "Stupid c*nt" or other really awful things. My mom has said that he is on a mental level of a 9 or 10 year old and that he has zero impulse control and just "Can't help himself."

He's done things I am not okay with and he scares me sometimes. There have been times where I've come home from school and he will habe his bedroom door wide open with his hands down his pants and he will just STARE at me. I've caught him in my bedroom GOING THROUGH MY UNDERWEAR DRAWER and I'm pretty sure he's taken my underwear from my room into his own. There have been times where he's walked around our house wearing nothing but his underwear and having an erection and it freaks me the fuck out sometimes. My mom is NEVER home enough to see him when he is like this. Now that we are still out for summer, amd my mom is gone pretty much all day at her job [she's a nurse and works 12 hour shifts most of the time] he just acts like he can do whatever he wants with no consequences. Whenever I try to tell my mom about these things, she just dismisses me and brushes me off. She's so tired after working she just doesn't care about his behavior. My mom literally has to yell at him to take a shower because he will go weeks without bathing or doing his laundry and he stinks. He is constantly posting on Snapchat about how much he "Wants a pretty girlfriend" and how he's so lonely and depressed and "hates his life"

I'm really afraid my cousin is either going to A. Get angry at his stupid games and take it out on me or B. Try to do something really gross with me. I don't feel safe around him but I don't know how to convince my mom about his behavior becausse he NEVER ACTS LIKE THAT around her. I still have another year before I can even think of moving out and I feel like I'm trapped with my cousin. I know he's family and I know it isn't his fault he has these issues + his parents pretty much abandoned him but I just wish my mom would listen to me and help me.

What can I do to make sure my cousin doesn't attack me? What measures can I take to protect myself? He has just gotten worse and worse over the years of living with us and my mom is too burnt out trying to provide for him and all of his medications and therapy to do anything else. Any advice will be so helpful.

tl;dr: Cousin with mental issues is getting increasingly more violent/doing weird sexual things. I don't feel safe around him and want to learn/know how to do protect myself. Help!!!!