Me (30F) struggling with past actions of my mother (40s)

Hello, long time lurker, first time poster. I'll try to keep this clear and not too long.

Short set up: I'm a socially awkward chubby fifteen year old girl in high school, my parents have just gotten divorced and I've been, in my mothers words, 'a rock'. My mother goes between praising me and saying cruel comments about my appearance, bullying me for not being able to make friends easily, etc. Never know which I'll get. I meet W (male) and K (female) at school and we become very close. My mother sets up my house as the designated hangout spot, she's fairly isolated herself and for some reason I don't think much of her talking to W and K a lot, setting up hanging out time with them on my behalf...

W seemed to clash with my mother a bit - he'd talk back to her/be cheeky and forward - which allegedly came to a head when my mother privately called him out on it. He stormed out and my mother insisted she had only brought up how rude he was to her. A few days later! I discovered the truth. My mother was insisting on a relationship with W. I realise now that slowly my mother was positioning herself as a peer to W and K. I was devastated about this. My mother told me to suck it up and made jokes about W being my new dad. K seemed to think it was okay although she did strike (!) my mother across the face when she was told.

My mother said me and my siblings would be put into care if I told anyone. The few people who did know (my stepdad, close friends of my mother) seemed to approve or at least not disapprove enough to take it any higher. W came out as gay a few months into the relationship and disclosed to me that he had been forced by my mother into certain acts he was uncomfortable with. He followed this up with telling my mother he had fantasies of killing us all, including her but she laughed it off. All sides demanded I say nothing and all sides were telling me things I couldn't handle. W and my mother were constantly on and off so I was forced into playing messenger at school where I'd have the brunt of W's anger and my mother would punish me for seemingly not helping her relationship with W.

At this point, my mother disclosed that my stepdad (who I believed was my dad) was just my stepdad. She told me all my friends (including W and K) had been told this by her earlier. To this day she refuses to discuss my birth father because 'she doesn't like talking about it'. Part of me keeps talking to her because I'd love the opportunity just to reach out to my dad's family.

I was a nervous wreck. I was hardly at school because of things with W and at home was no better. I think I might have had a nervous breakdown. Me and K became closer and ended up dating though she pulled back without explanation, my mother said she'd intervene (!) and I discovered them together, well. Yeah. W was permenantly out and K was in. My mother demanded I respect her as a stepmother, would laugh at any sexual comments K made about me and they made fun of me being depressed. There's a ton of other dysfunctional shit as you can imagine...I was my siblings main carer at this time, I put them before any decision I made because my mother wasn't willing to do so.

Eventually, at twenty two I got out of there. My mother and K split up after six years when my mother cheated on her. She still brings up W and K if she sees them in town (W is NC with her and K talks to her, her partner is unaware of what happened) and doesn't see why that upsets me. I constantly think about reporting what she did, K probably sees it as consensual so I'm not sure what good it would do. My mother says no-one would believe me and no-one cares. I think what she did is wrong but after so many years and adults knowing but not saying anything, I wonder if I'm the crazy one?

I've tried to cut contact with her but always get pulled back, my sister (18F) just wants everyone to get along. Everyone seems to have the attitude of 'just let it go'. Am I being horrible and petty? My self esteem is non existent and she goes out of her way to say she did nothing wrong, that she was depressed. I don't think depression makes you do things like that.

I hope this isn't too disjointed, I've never disclosed this to anyone else before but I can't carry this anymore.

TL:DR - toxic mother had inappropriate relationships with high school friends while emotionally/mentally abusing me and neglecting my siblings. Everyone says to let it go but I can't