You can either stop doing stuff for them.. and end up in a huge family fight..
Or you can basically decide to just let it go and let them continue to spoil her and not make her pay for anything.

It's a family thing. If you're living outside the home in your own place, then you can just make certain not to assist with things they want you to do. When she took your car you could have pressed charges at the time.. but you chose not to.. trusting your parent to pay for the damages(whether she said your sister would do it herself or not, as the adult, she took on the responsibilty of the bill). Since your parent seems to have no intentions on paying the bill, you can either try to go to small claims court(not worth it in so many way) or just suck it up and ignore it.

I'd say move on.. and stop doing things for them. If they can't uphold their responsibilties to you, don't do things for them.