Fraud against elderly parents

A few years ago my elderly parents helped one of my sister purchase a new car. Because of her unstable job situation and no real credit the vehicle was in my parent’s name. Needless to say they have been forced to make payment for the last six month to prevent default on the loan. Fast-forward to the present… she returned to the same dealership (w/o my parents) and traded the vehicle in for a much more expensive one and the salesman/finance people allowed her sign their names without any consent from them. The only way they found out was when they received a call from the bank asking them if they were enjoying their new car.

Where should I start in terms of getting this rectified? My ultimate goal is to get the dealership to either remove my parents name from this loan leaving her sole responsible or recalling the vehicle and canceling the sale.