Jess I can be reached at 405 458 4272.
I may be able to assist you.
What I am Gathering From the information and your statement is, that you guys were on private property, so the police usually won't come out and determine whose fault it was or whose fault it is not. However the car that hit you if it was their fault is responsible for your damages the insurance company has sent you a letter trying to avoid paying anything and making you fight for it. Typically the insurance companies today will not just do the right thing but will make you earn it my suggestion is that you send them a demand letter or possibly hire an attorney and give them a couple of $100 to write the letter for you ultimately if it was not your fault and the other car backed into you the car's insurance is responsible them telling you that the driver was excluded just means that the owner of the car is responsible for the damages which ultimately the insurance company will have to take that up with the owner of the car.
So the owner of the car would have to file theft or unauthorized use of a motor vehicle, unless he actually gave his nephew permission to operate the car. Otherwise he is responsible for the damage because the person was or had permission to drive the car. I have provided my telephone number up above if you would like to discuss further reach out to me