I'm assuming the car is in your name, right? I'm also assuming you only made a verbal agreement with your father.

I think you should write up the agreement you made with him on a piece of paper. Spelling out the exact deal. He paid your tax, you let your sister use your car, how long until you get your car back. Put specific dates on these terms. Get your dad and you to sign it, even get a witness to sign.

If your dad refuses, do you have a spare key to your car? Just go take your car back. It's in your name. You can even report it stolen and get it back if you have to!

I don't care if they are family. Your brother is shaking you down for his drug money. Always get everything in writing.

Of course you aren't giving your brother any money. Tell him 'nice try'.