How do I get rid of stalkers that are related to me? In this case, my parents.

So my parents are crazy. And I know a lot of people say that but I actually have crazy parents. They are prescription pill poppers. My mom is a religious fanatic who believes that she cannot stand up to my father because God said he's supposed to be above her. My father is verbally abusive and controlling as all hell, mainly towards my mother but also towards myself.

I moved out as soon as I was eighteen (and they couldn't drag me home using the police) right after they accused me of being depressed and suicidal and forced me to get a on a lot of antidepressants and sedatives (which they actually took from me on occasion for themselves). So I left but maintained contact because I have a younger sister still living with them who's not really aware of their true colors yet and I want to continue to have contact with her.

While I was living with them my parents regularly listened to my conversations, followed me in their cars and demanded my pay stubs to see if I was actually at work when I said I was. They would then have seemingly friendly conversations with me where they would ask general questions like "how was your day?" but expect very specific answers. If I didn't tell them what they had seen I would be grounded. Anyhow I've been putting up with all of this for my sister's sake but now I can't take anymore because I found out my dad is one CREEPY fucker. Not only is he totally and obviously addicted to porn (caught him at it several times, he doesn't close the door even though he knows I have to walk by to get to my room) he's also wandered through the house in the middle of the night, and I suspect he was touching himself while I was in the same room as him watching tv. Needless to say, the man is a pervert.

And if that weren't enough I've come to realize the sexual abuse I went through as a child was at his hands. I always knew something had happened but never knew the specifics or who. Memories came back and I can't stand to see him anymore. I need to cut ties, but I'm not sure how to do so without getting a horrible result: them stalking me further or even just blocking my driveway so I can't go to work and demanding to know why I can't have them in my life. What do I do?

TL;DR My parents are insane (examples above) and I have to cut ties with them with as little repercussion as possible for the sake of my younger sister who still lives with them.