I'd do it. We've all done stupid things as a teen. Unless something is particularly dangerous like he's drinking and driving or doing hard drugs or something, I think we gotta cover for each other now and then. Who buys us beer or gets us fake id's if not our older siblings.
I know I sound awful to some of you- especially those who are parents. But the thing is you aren't the parent. You're the sister. It's how it is in real life, not the Dr. Phil show..
In My Humble Opinion, it's in the Sister's Code to help him out.