Gotta respect their decision. I mean, I see my father falling apart and if he made the decision idk if I'd contest it. He's not able to really go out, has trouble breathing, etc. Staring at the same 4 walls all day until you die naturally isn't the best way to go out. A little bit of you dies mentally every day going through that until you're just old, depressed, and unable to function. Why put them through that misery if they don't want to essentially rot in a bed or chair.

Don't feel guilty about this, or as if you and your brothers should have done something differently. It wasn't your choice to make-- it was hers and your parents'. They may have understood her condition and wishes differently, and had conversations you weren't a part of. In the moment, you guys didn't challenge the hard decisions your parents were making, and instead showed support. Which is doing something in its own right. Moreover, morphine can give a pleasant high, and combined with patient's own adrenaline surge around the times they're the sickest-- can make very sick people look much better. Consciousness/lucidity waxes and wanes a lot in sick people.

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.