Under the laws of all states in this country, unless the buyer of the car that your sister purchased disclosed to her that there were $800 in fines associated with the vehicle she purchased before she parted with her money and such was a consideration in the amount paid for the vehicle, the seller is responsible for the $800 in fines.I suggest that your sister pay the $800 to get the car registered and then sue the seller in small claims court if she can afford to do so.