That far out you are seriously tempting fate that both your grandparents will still be alive. .
No, seriously. Not to be grim, but in the past year alone, seven —- SEVEN — of my close friends all lost parents. Most were only in their late 70s. A year ago? Nobody was even sick. Now, they’re all dead.
Hinging your entire wedding date on the health of anybody is unwise. It can make for a very grim day if the unthinkable happens.
You can honor your grandparents no matter what day you marry. Why not do so five or six months ahead of the sister. That’s what I’d do.