Either let my brother borrow my car or don't go down to the Cape for the weekend.
What the hell?
I would say, "Fine, Mom, I'm not going to the Cape. And it's your fault that I don't get to enjoy this weekend."
Seriously, can you find all the keys to your car, and take them into your custody? Take them out of your purse if you have to.
Or better yet, drive yourself with your car. If Mom wants to leave a car behind for brother, let it be hers.
He's not allowed to borrow your car. Full stop. Even, frankly, if he's stuck with no transportation.
Ye gods! All your reasons are incredibly good ones. The last thing you need is a smashed up car, or an impounded one (if he's got illegal drugs in it when he's driving).
And I agree w/ PastryGoddess--you don't need your mother's permission for any of this, even if she is helping to pay for the car.
Let me make it clear. I have little doubt I can win this argument (getting to go down to the cape with my mom without my brother borrowing me car), but I would rather avoid the war it is going to take to get there.
I think it might be worth the war. And in fact, I think it might be worth it for you to raise the stakes. As a parent, I finally figured out that the BEST thing to do was to drop the WORST punishment right away. Find something that would really, really make the kid cry. No half measures. If there's going to be a battle, I made sure I won it, and I did it by going nuclear right away. Then the conflict was over, and we could move forward without power struggles. It actually meant there were fewer battles, and fewer hours of *both* our lives spent in unhappy conflict.
In this conflict, you are the one in the right. Absolutely. And so you are entitled to win this battle, because you are right.
So, this time maybe you should be the one who takes it nuclear. Maybe you should be the one who gets really, really mad. (It's not rude, actually, to be mad. Really mad.) Figure out, as if you were the parent, what are the consequences that would make her most unhappy? That would make her most upset? Then drop those. So maybe you silently pick up your book and leave the room in a stony, disapproving silence, and you go to your room and refuse to speak to her for 2 days. Or maybe you leave the house. Something that says, "I'm the grownup and you are out of line."
Maybe you feel a power imbalance because you're living w/ Mom and she's helping to pay for stuff. But this car is in your name, and you are responsible for it.
You might also try the "cut and paste." Two sentences that you simply repeat over and over, without ANY variation (except maybe getting a bit more exasperated) every time she brings up the topic.