I'm the youngest of my siblings too. I think the youngest never gets a whole lot of respect no matter what but I also think working or not working makes a difference; "professional" titles and jobs can make a difference for some reason with parents (and others) too.

You are eventually going to lose your parents so yes, I'd work with a counselor toward that day. I found it very stabilizing when my stepmother was dying/died to have a therapist to talk to. I'd also get a job and move away from my parent's house since it doesn't seem like you can help having this sibling around. I'd try to disengage from their care (especially now when it seems they're doing a bit better?) and/or maybe try to get them interested in an assisted living situation so you wouldn't have to worry as constantly either about them or your sibling's meddling. I'd give up on the car thing or install a theft deterrant lock on the steering wheel like "The Club"? Some extra lock that couldn't be duplicated. That would certainly get the message across. Too bad at the time you didn't think to at least ask her for $5 for gas or something, not mean but getting a message across that it's your property and costs money to own and operate.