Just a thought.
You've been away for a while. Worse, (I presume), you've been outside the US, among the primitives of the rest of the world.
You go back now and announce this major change, they might think you've been infected by some anti-American thingie.
If I were you, I'd take it gradually.
You mom sounds good. you don't say how you think your sister will react, so might be best strting with your mom.
Introduce your partner. Then when that settles down, tell her about your plans.
As for your grandparents, that doesn't sound like it's going to happen really. Dementia is a strange disease. In Altzimers, the recent memories tend to go first, gradually moving back.
The other main type, things tend to deteroiate in bursts. This use to be called multi-infarcaction demetia. Not sure now. probably has a really cool name which doesn't mean much. (Progress eh?).
But one point. Don't be frightened about your life or your own future. This is your life, your future. Like the rest of us, you just have to make the best of it.