The grandmother is at fault for what happens because it is she who insists upon visiting the house that she is "sure" to be down the road they take. If it were not for the grandmother, the family would not have (a) driven down the road or (b) the cat would not have jumped upon Bailey, causing him to lose control of the wheel and have a wreck, or (c) the Misfit may not have felt it necessary to kill the family without grandmother's verbal expression of recognition of him as the Misfit.
I would agree that the grandmother is an instigator -- catalyst -- of key actions, as listed in Amy's post. I would also take the question a step further. Are we asking, Who is to blame in terms of the family running into The Misfit, or are we asking, Who is guilty in this story?
If we are asking, who is guilty in this story, let's examine some other characters. Bailey, the son: how good of a son and father is he? June Star, how good a little girl is she? Red Sammy and his wife: are they good, honorable people? We could go on like this, taking an inventory of each character, and find that no one is particularly admirable (I'll let you find the direct evidence). O'Connor wrote from a spiritual, moral perspective where "all have fallen short" (a paraphrase of St. Paul's letter to the Romans) and thus have sinned. I would encourage you to make a list of each person's sins and then analyze the story from a more global, moral perspective. While we tend to write off The Misfit as pure evil, isolated, an alien, is it possible that everyone is to blame for something they did wrong?
This is no argument for equating sassiness with murder, but rather, my challenging you to ask the question on a bigger, more thematic level.
Most probably the grandmother's. She is the one who is insistent on stopping at this place that she can't seem to remember correctly how to get to when there is a convict on the loose. It is her self-indulgence and the confusion brought about by bringing the cat, forgetting the way to this place, and taking the vacation in the first place. Had any of those things been different, the family would not have been put in the situation it was, and no one would have lost his or her life.