Be a respectable person in every regard. Set an example for your sister, make her respect you. You'll be her role model, she will want to be like you. You won't need to tell her what to do, and if you do, she will listen to you.

Also be nice to her. Don't be indulgent, but be nice. You know?
She might be "special", but that can never be an excuse. However it might mean it will be easier to go about parental guidance in a different way. I have a younger brother with aspergers. He generally respects me and listens to me when I tell him something. But in some areas he's a bit slow to catch on, and he likes to use his diagnose as an excuse.

Still, he could be much worse of, and I know it's largely thanks to good role models that he isn't. And for the record I don't feel your parents on their approach and I understand your frustration. But since your situation is thus, you'll have to tackle it by yourself.