Your neice can sue her father's estate since he was at-fault in causing the accident. She woud have to sue, if the estate will not voluntarily pay this: your brother caused the accident, but the car was not his--he did not owe the balance on it to the lender or dealership; she did. Since he was not the one directly responsible for the car's cost, the estate would only have to pay if she sued it, the way she would sue any other at-fault driver who damaged or destroyed her car, and proved the liability for the accident in court.