I'm a bit on the fence with this one. I think its completely fair to expect an 18 year old to help out a bit. But I'm also mindful of the unfairness of pushing parental responsibility onto older siblings.
I'm also conscious that I didn't have it so difficult as I did at 18 until I became a parent. Other people will have other experiences, and I'm sure someone will do the Yorkshireman act about how the raised her 6 younger siblings, did their nappies etc, took them to school, then did a nightshift 7 nights a week and got straight A*'s all with a smile on their face. Bully for them!!
Juggling A levels and the associated exams, homework, coursework, revision, with a job, plus an expectation to help around the house. An an expectation that you're supposed to be an adult now, but teachers and family don't treat you that way. And god, forbid, you try to have some kind of a social life. So much pressure and very little time to scratch ones arse. That couple of years with A levels and a job is quite a challenge. She does run the car herself which means she must do quite a few hours given how much car insurance costs for an 18 year old, and an 18 year old part time worker is hardly going to be raking it in.
I would try to be a little less confrontational, explain how such a small action from her takes a huge amount of pressure from your shoulders, and it frees you up to do the things that supports her, such as meals, washing, etc. that she might otherwise need to do for herself. Don't phrase it as a threat, but explain how her help ultimately benefits her.
And I'd cut her some slack on the perceived attitude. Like I said, 18, doing A levels (properly, and fulfilling ones potential) while being employed to the extent she can run a car on shite wages is not easy, and being a stand in parent is a step too far.