Will repeat myself. If you disagree with your parents' decisions, have you documented your own advanced directive plan? If you don't agree with what they decided for your grandparent, highly likely you would not agree with their decisions if they had to make them for you. You could be hit by a bus tomorrow and unable to talk / write your desires down. What would you want if you found yourself in a hospital bed without a decent shot of getting out of it?

Get it documented, make it legal, and if you don't trust your parents, then declare somebody else able to make medical decisions for you.