Just park in the driveway and when they question it.. just say WOW HOW DID THAT GET THERE!?!?!?! :P
Better yet, say to them, I bought the car for you guys. When they say I don't want it.. ask them, Well can I have it? :(
Just say you really want the car for the parts. It's got what you want on it and what you don't need afterwards, you can sell in an effort to get some of your money back. What more can you say, dude? Just be honest with em, you're young and you are family.. they forgive you eventually and even if they beat you with a stick... you'll heal fast with youth on your side :) just kidding dude :(