has anyone been through this before? i need advice.

okay so for the past year i’ve been focusing on my mental health and trying to get better, i moved to online school, got my meds, just genuinely trying to get better. obviously i was worried about the distancing and friends because most of my school ‘friends’ stopped talking to me once i left. this really left me with one friend who’s my best friend, we’ve been friends for around 3 years now and just grew really close, she’s about 2 years older than me but were literally just so close. I have an older brother who is also around 2 years older than me, obviously she liked him a bit so they started talking and what not, they grew a bit closer and he decided he wanted our parents to know and her parents which is obviously something me and her were kinda against at first because she’s been my best friend and we didn’t want them thinking anything weird of it. I can’t lie when i say i think hes a controlling manipulative person when in relationships, i let that be known but it was all fine in the beginning, with his past girlfriends he wouldn’t really like them trying to be my friend and stuff so before they even got together my mom let him know like we’ve been best friends and he knew we’ve been best friends so that was that until he started bringing me into there arguments, trying to blame me for some of their arguments. It just got worse till the fact he started blocking me off his & her social medias, arguing with me & calling me names, and accusing me and her of being SEXUAL. i brought this all to my mom and she tried having a deep talk with him but he’ll even argue and say slick stuff to my mom but eventually he agreed to keep me and hers friendship apart from their relationship. about a week passed and i went to his room to get my dog, he flat on ignored me through the door while i was saying his name and could hear him on the phone i went upstairs to tell my dad to get him and that’s when he sent me a message saying “see this is why niggs don’t give you sht.” i don’t even know what that was supposed to mean because i wanted my dog but he said that and proceeded to block me off of my bsf account when it had nothing to do with her, so i ft her and she confronted him and he got mad that i even called her and a whole bunch of stuff like that. but some other messsages he sent me were things like “kys” and when i tried telling him to stop blocking me off her account he said “honestly shut that shit up u can text her on imsg.” he once argued with me for like 3 hours while me and her were upstairs waiting on him then he said it was my fault he didn’t have time with her. He’s just a really big manipulator and gaslighter and i honestly think it irks him that i don’t put up with it & call him out. my bsf has tried ending things and he says no he wants to fix it but never tries..i just don’t know what to do and need some advice.