C.B.
Oh this sucks. I hate hearing these stories but they happen all the time.
First, I wouldn't send Sister the title to your old car until this is all resolved. Second, if you haven't already done so, file a Release of Liability with the CA DMV on your old car. I'm guessing that the car may be currently uninsured and if Sister has an accident, you may be held liable as the registered owner of the car.
As for Cousin's car, I think a major point would be when did he sign the car's title over? Was it before or after he was declared incompetent? If it was before, you should be okay. Contact the cousin's lawyer to see the exact date he was legally declared incompetent and compare it to the date he signed over the title.
I agree with the other post who said let them make the arrangements to trade vehicles. They can pick up the guns at the same time. You already flew at your expense to pick up Cousin's car and drove 700 miles to deliver your old car. I think your financial obligation in this matter is over.
The only reason everyone is making a fuss over the car and guns is they have a monetary value and are easy to sell. I doubt anyone is fighting over his old recliner.
I find it amusing that his sister said had it not been for Cousin's mental illness, everything would have gone to his son and you both wouldn't be involved. Yet now she is going to give the son HALF and keep the rest??? If she truly believed what she was saying, she'd give the son the whole estate.