To show that the grandmother is a misfit in the story, if that's how you see her, you could pretty much choose any line in the story. I don't see her that way, since she is the abuser of the bunch. She creates misfits, in a sense. She isn't the one who doesn't fit--she determines, obnoxiously, who and what fits.
But if you're already committed to the essay, you could start with the opening paragraph.
The narrator writes that:
The GRANDMOTHER didn't want to go to Florida.
So she concocts a story, rationalizing a reason not to travel south. She asserts that she would never do something her son is about to do: travel south when there is an escaped convict on the loose. Here's her manipulative rant:
"Here this fellow that calls himself The Misfit is aloose from the Federal Pen and headed toward Florida and you read here what it says he did to these people. Just you read it. I wouldn't take my children in any direction with a criminal like that aloose in it. I couldn't answer to my conscience if I did."
I suppose you could attempt to prove she is a misfit, because she is the only one of her kind in the story.
I'll let other editors cite other quotes for you.