M.H.
Well, I would be pretty firm that she WILL go. Set an egg timer for every 30 - 45 min. When that goes off, stop her movie or her play time and make her go. This will get old with a girl of her age fast, esp. if she is busy doing what she wants. Everything should stop for the 10 min that she sits on the potty.
Tell her, if you can go accident free for three days, I will put the timer away, but if you have accidents, it comes right back out. IT doesn't have to be a power struggle, just a no nonsense, this is how it is. Stay calm, excort her from her game/movie to the bathroom, and when and if she groans when the timer goes off, remind her that if she decides to start listening to her bodies needs, you will get rid of the timer.
just a suggestion.
Bribery, ... ur um .. incentive always worked well here too! :)
Consistency is key, whatever you decide to do needs to be implemented when you're not in charge as well.
Best of luck, and kudo's for being such a wonderful grandmother!!!