Sounds like a tough situation to overcome. My advice is slow and stealthy (but not disrespectful). Look for small tasks to do for the car. Things you could get your sons to help with. Then, when visiting your mom, maybe the boys suggest "fixing" something on grandma's car.
Maybe at first it's only cleaning something, but a little work here, a little work there, but always with the boys, and who knows how things might turn out.
