Just because they treat you like you're 17 doesn't mean you have to act it.

If your sister was wise she'd loan you the car anyway. One rebellious child in the family makes them look sympathetic. Two rebellious children makes the parents look like monsters. Their friends will all ask about you two in the course of conversation and what will D&V reply? That they've cut off both of you because your sister wanted to loan you her car? Now THAT will make people talk about them and not in the way they'd like.

This will ultimately damage them in ways that matter to them.

Let go of what you cannot change and don't stoop to their level of maturity. Things are replaceable, hearts are not, and I don't mean theirs.

Keep an eye out for what happens. This isn't over yet.