Hi Nat!
Wow, that's a super sticky situation your family is in. Unfortunately, it's definitely up to the adults on this one.
And by the way, I know how totally gross houses can get when people smoke inside- some of my relatives do this, and their house lowkey disgusts me. At least the smokers I live with only do it outside on the porch. Ever.
And although your grandpa does smoke, don't worry about getting cancer- you won't! I have a long line / family of smokers, and I've been around it my ENTIRE life, and so far, no one has ever gotten cancer, including the smokers! So although second-hand smoke is totally nasty, it shouldn't harm you too bad.
I really hope your family can work this out and come to a compromise!
Sending prayers <3