If she is about to pass on just take the car and if its anything nice keep it for Shelby, or just sell it and use the cash as needed.

No since in trying to prove a point to her now.......

You saying "thanks but no thanks" will not bother her one bit, I mean she did not give you a car back in the day like the other two siblings got and it hasn't really bothered her till now. Do what ever you have to, so she will go to her grave happy.

Take the car, sell it, that's what I would do.