16 year old autistic girl in a very complicated situation, please give advice/help?

Before I begin I'd like to say I live in Ireland, specifically the North area, so I'm under UK law, if any advice I'm given aligns with legal means, please keep this in mind as laws may be different here.

I'm a 16 year old girl and I have autism and dyspraxia, this will be somewhat important; for my entire life, my younger sister (13 years old) has tormented me and intimidated me, verbally and physically. She was rather spoiled when she was younger, and it has definitely rubbed off on her as she's grown.

She yells at me a lot, even when I've done nothing wrong-- even if I just look in her direction, I get ridiculed, every little thing I do is judged, she screams, she fights, she calls me horrible things, says she hates me, has threatened to kill me, says she wishes I was dead, etc. I am underweight and around 5'3-4, she's a little shorter and skinnier than me, but way stronger. The day she threatened to murder me still terrifies me.

My parents don't always get along, but I'd rather have that than what's happened over the weekend. It came to light that she lied about a girl in her school groping her 'down there,' which she told us last summer, I believed her, but it turned out to be totally false and as a result she lost all her friends- I just assumed they were fake sucky people. Recently, she lied to the school about my mama groping her chest. This is totally false, she said my mama grabbed her chest in bed; she and my mama shared a bed when she was little and my mama had laid her arm across her chest while asleep, she did not touch her.

She is very manipulative and malicious; she was taken to the doctor today and when she thought she was in trouble, she was shaking and crying, the moment she realised she wasn't, she immediately calmed down and was as bright as day from what I heard.

My big sister, who is 17 years old (we are 1 1/2 years apart,) is able to drive and has many friends and a boyfriend. She owns a car and has a job, she can escape. I can't, I'm disgusted with my younger sister, I always thought she had a mental illness/personality disorder based solely on her treatment of me, but now I feel hurt that only now my parents are thinking something when I've said there's something wrong for years.

School is my only escape, I can't go out after school and visit friends that often, I don't want to be trapped in my room. My parents are both my parents and caretakers regarding my disabilities, I'm scared and I don't know what to do.

TL;DR: I'm living with an abusive/possibly mentally ill younger sister who accused my mother of sexual abuse and I do not know what to do.