Depending on how old you are, and whether you are financially dependent on them, I would tell them to go fuck themselves. I’d also tell your grandma the world is changing, she’s gonna die soon, and there’s nothing she can do to change either one. Flip them the bird and walk out.
Obviously, personalities are different. This might not work for you, but you can’t just fight back with facts because they refuse to play that way. If they want to throw this kind of shit your way, you have no obligation, nor should you, play by the rules of “they go low, we go high”. That’s always been bullshit anyway. Especially in a situation like this.