Grandma won't live forever- that is pretty bad. Neither will whoever told you that, though it's kind of mean to bring it up in a conversation.
I once told my former MIL that "guilt trips only work if I give a f@$k about how you feel." Not very diplomatic, but it worked. Some people can't abide a simple "no thank you", and silence just leaves them room for nagging and manipulation...
Or you could bite the bullet and go, but leave an upper decker in their bathroom.