The grandmother always had something to talk about. It could be stories from her childhood, things she saw on the news or heard on the radio, or it could've just been she liked the sound of her own voice. The point of downfall for the grandmother is when she influences her son to take a detour to go see an old house she remembers. As the family rides down the back road she suddenly realizes the house is Tennessee. The family is in Georgia! Her reaction to the realization of this started the entire ordeal that would become the fatal mistake made by her.
The messed up thing of all is that the entire family is killed because the grandmother refused to talk and talked too much at the same time. The family never knew why the cat reacted and jumped onto Bailey's shoulder. The only reason that we know the family is killed is because the grandmother recognized the Misfit. It seems that if the grandmother would have kept her mouth shut but speaking would have saved the family's life.