Don't just automatically give up.
You should be getting another opinion from another probate attorney. The attorney will probably know where to look to see where and if the will was filed or at the very least could question the coroner to get him to give more details about what he knows..
It looks as if this sister has deliberately withheld filing the will, which is most improper and probably illegal. Your attorney can file to be adminstrator of the estate (sister's mistake was in not filing herself to be administrator--she has no authority to dispose or handle any of the assets since she is not the administrator, but she probably doesn't know that) and then get the car awarded to you.
Administrator can also ask the judge to order sister to produce the will. You should hope and pray that she still has it--if she doesn't, then whatever is left in the estate is going to go to this man's wife and children, if he has any, and if he has neither of those, then it's going to his children. Even if sister has sold the car you can file suit to show she did it without legal authority and she must either get the car back from the person she sold it to and refund their money, or she owes YOU repayment for the value of the car.
You say you "were listed to get the car after his death"--listed where, in the will?
DANDY DON IN OKLAHOMA