Who is Brynna and Katie from Waiting for Normal and what is their importance?

Brynna, 6, and Katie, 3, are Addie's little sisters. Addie and Dwight refer to the girls as the Littles. Addie loves her sisters and is devastated she can't live with them all the time. The girls have lived with Dwight since Denise left them home alone for three days when Katie was just a year old. Bryanna and Katie love their sister. Bryanna suffers from some emotional problems due to their mother's behavior.