I hope you choose whatever feels right to you.
I am protecting my grandparents from knowing the shit that my parents have done because it will shock them majorly and I'm not sure if it's worth it. There's only 1 living grandparent left, he's half deaf now and I barely speak with him at all (we live in different countries so I never see him and don't have much of a relationship)... so I am reluctant to stir up the drama.
Especially since it'd just make him worry and he won't be able to do anything, nor fully understand what has gone on.
But if everyone's deluded and you want the truth out there, then I hope you feel free to say so. Just be careful what you choose and any repercussions. Her health may actually be okay if you have a gentle and serious talk rather than shock her or something... I don't think emotion-related strokes or heart attacks are common at all unless you're scaring them in a "boo!" kinda way (=> heart accelerates => heart attack or stroke). But be prepared for them not to take it well, or for more family dramas or whatever. But on the other hand, at least you'd be being honest.
Do you care about what your nan thinks or do you have a relationship with her? If you don't maybe it's not worth ruining her peace. But if you do maybe its something you wanna tell her before she passes.
it's lonely being narcissistic