Yes, you are right to comment on this, because, as far as we know, the grandmother only has one son, Bailey. However, at the end of the story, as she converses with the Misfit whilst her son, daughter-in-law and grandchildren are being killed, she suddenly has a kind of epiphany which involves her identification of the Misfit as being her child:
Why, you're one of my babies. You're one of my own children!
This is a statement that must not be taken literally. Rather, if we look at this quote in context, we can see that the grandmother is tremendously moved by the Misfit's expressed desire to ascertain the truth of Jesus and his actions. Her epiphany is therefore based on her understanding of a sense of human connection between them both and her awareness of the similarities between them. She realises that she is a "Misfit" just like the Misfit, but just before her death she is given this moment of insight in which she is allowed to see herself for who she really is.