What are your rights if you paid a relative's auto loan off and have possession of the vehicle?

In all states the way the vehicle title is Worded determines the ownership. Even when someone pays off a loan, unless their name is on the title they do not own the vehicle. The person who the vehicle is titled to has the legal right to recover it from whomever may have possession. The party who paid the loan would have to file a lawsuit to recover their expenditure if the vehicle owner would not voluntarily submit repayment.