Selling a car whose odometer was rolled back would, at a minimum, be a breach of contract even if it was innocent on the part of the seller (he or she didn't do or know it). It could easily constitute fraud (if the seller was the one who rolled it back). There should therefore likely be good grounds--assuming the report  is correct--to either obtain some compensation or possibly rescind the contract. Your sister should consult with an attorney, who can advise her as to her rights and possible recourse. There may also be a statute in your state providing additional damages (compensation) for this specific type of act--some states have them--so that is something worth exploring, too.