If you do something illegal, your aunt and uncle could lie to make it sound worse. Then you might have to give them the ATV to get them to drop the charges.
Remember, they don't have to lie to make you look bad--by doing something illegal you will have already hung yourself (and being awfully close to 18--the court systems might try the crime as an adult).
In the end--don't ever consciously do anything you know (or suspect) to be illegal. With databases today--even little things get remembered forever and will restrict your ability later in life to make choices (like jobs in government, police, military, security clearances, etc.--and who knows what decades from now--for example a cop who had a domestic violence complaint and never fought it because the fallout was so small years ago--can loose the right to carry a gun now).
You, learning from minor mistakes, and acquiring the ability to move on from those mistakes and how not to escalate the problems, will serve you well in life.
Look at your relatives, they stiff you, and now appear to be doing the same to the mechanic. They have not learned, and will be the worst for it in the long term.
-Bill
By the way, another reason it may not be worth following up on this--the mechanics lean on the vehicle will probably have to paid off before you can reclaim the vehicle. If the relatives don't have any money worth attaching, you can win and still come out with zero results (other than the fees and time from small claims court).
