Zingy,
You have my empathy. My mother died a little over a month ago, and she also had Alzheimer's, although not in the final stages. Perhaps your grandfather got a "living will" before he developed dementia, and that's how he has the power to turn down food. My mother got one years ago that said no extreme measures were to be taken to preserve her life. Regardless, try to make up with your sister while you're there. It's odd how losing someone can change things. My father and I didn't speak for well over a year before my mom died, but we're now on friendly terms (well, as friendly as they'll ever be). Anyway, I'm so sorry to hear about your grandfather. I'm grieving myself, so if you'd like an understanding ear, please send me a PM.
Take care.
Suzanne