Is she still providing excellent nursing care for your parents, and do you have other options if she stops providing it? Yeah, she's taking advantage of your generosity.... but ....
Everyone's situation is different .... my parents' care would be most important, and a car I could afford to "write off" would not.
But I'd still be pizzed off, but just wouldn't let it upset that parent situation.
And NO good deed goes .... well... you know
Just my .02 worth...