My sister pulled a gun on me

I'll keep this short because there are just so many different components. I 27F am being emotionally abused by my 29f sister. I'll refer to her as X Two years ago, things were good for the most part. My sister and I got an apartment together. We both had boyfriends at the time. Her bf only came to our apartment once with her for a few minutes. One day my X and my mom had me sit down and X revealed to me that she believes I am having a secret sexual relationship with her boyfriend. X told me her reason is because a few months ago I mentioned going to see a new movie that just came out with my boyfriend. And her boyfriend had suggested that they go see that same movie for a date night. I assured her that it was simply a coincidence hence a popular new movie and it was a Friday night. Fast forward, X has accused me of sleeping with her boyfriend for the past two years. X and her boyfriend broke up after like 6 months of dating. X has accused me and lashed out in violence on several occasions even to this day, long after that relationship. X would apologize and insist that she has paranoia and will seek treatment. X dated another man for a few months and also accused me of sleeping with him as well, she apologized and chalked it up to paranoia. X is now seeing a new guy. Yesterday X asked me to come to run an errand with her. While in the car, she pulled into a dark alley and demanded that I surrender my phone because she's convinced that I am sleeping with the new guy. I asked why she thinks this, she began shouting that she is sure that I had sexual relations with all three of these men, and that she has believed this all along. She pulled out her gun and threatened to "blow my brains out". I got out of the car and walked back to my apartment. Today I am moving out back with my parents. I don't know what to think or what to do. I'm still dating my boyfriend, X often times will text him while I'm with him and it's extremely painful. I do not understand what type of mental illness she has. I am afraid for my life. Help, advice please. I've told X I am going no contact with her but I'm sure she'll find a reason to come to parents' house she has a key. Help