[@Wick] It's... it's just the worst, isn't it? What sucked most, I think, (aside from losing my granddad long before he actually died) is that Dementia isn't an illness, just a symptom. We didn't even know what he had until they did an autopsy on his brain (turns out it was Lou Gehrig's disease and also Alzheimer's, among some other minor problems) so we couldn't treat it. And he was so healthy! There wasn't a thing wrong with him before the Dementia - he didn't even wear dentures! He was well into his seventies when he got it! And it acted so quickly, within the space of like four months he went from perfectly fine to totally gone. I'm telling you, I'd rather die than have something like that happen to me, and put my family through that.  

Sorry, I have a lot of feelings about this (and don't even get me started on smokers...)

I'm so sorry this is happening to your mother in law, and I'm so sad for you about your grandmother. I hope you and your family can stay strong and find comfort in each other through this difficult time.