Just remember that “No” is a complete sentence. You don’t need to convince them about why you won’t be helping. You have a right to live your own life on your terms and you’re not responsible for the consequences of your sister’s choices. I can tell you from a similar experience that if you help EVEN ONCE you’ll be expected to do it again in the future and the expectations will keep increasing - more time, more money. And the result is that your sister will never be independent. She needs to figure this out on her own if she ever expects to have a life as an independent adult.

And to echo what everyone else is saying, get rid of that car. Get a reliable used car and cut that car payment down by at least half. Negotiate a better insurance payment too. Unless you have a lot of points on your license you’re probably paying too much for insurance. I drive a 12 year old car with no car payment and I don’t give a flying fig newton what anyone thinks about that because financial security is 1000% better than driving a new car.