The pair of sisters
The minor characters, Aunt Lily and Aunt Nona, provide a foil for the pair of sisters. Lucille and Ruthie end the novel by going their own way, but their adult aunts show that life can bring sisters back together. Otherwise, they don't serve the story very much, except to hand the kids from grandma (who dies) to Aunt Sylvie. They represent the potential friendship that might arise from Lucille and Ruthie later in life.
The opposite temperaments
When the mother of these two sisters committed suicide before the novel begins, the two girls have an orderly life, more or less. Their house is orderly and appropriate, and their expectations for life are straightforward. But, afterward, they are launched from their orderly world into the chaotic reality of death, loss, and human suffering.
The two opposite temperaments indicate two paths for people who suffer. Some will prefer the more chaotic path, because the chaos around them aligns to the chaotic emotions within them, and some will prefer the orderly path where they can depend on structure.
The suicidal mother
The mother symbol in this novel is obfuscated by the mother's suicide. Typically, a mother represents potential, because a woman's body brings forth life and nourishment. However, in this novel, the mother represents death, horror, and the hellscape of severe depression. Just like kids inherit their parent's money when they die, these girls inherit their mother's agony when she decides to take her own life.
The motif of change
In the novel, the custody of the girls changes no less than 3 times. First, they are in the care of their grandmother, then they end up with the pair of aunts, but the aunts hate Idaho's cold winters, so they recruit a third aunt, crazy Aunt Sylvie. These changes form a motif that underscores the constant change that trauma brings into their lives. They don't get to feel "stable" in the novel, and Ruthie goes so far as becoming a vagrant with her aunt.
The motif of mental health issues
The mother commits suicide, indicating severe depression or other mental health issues. Then, we meet Aunt Sylvie who is unorganized and impulsive, likely a traumatized individual herself. Ruthie develops social phobias, and Lucille develops a fear of change. These lead them into serious despair, and it's important to remember that many of the genetic issues that contribute to serious depression are hereditary, so these girls probably also have whatever mental health issues their mother dealt with, especially in the aftermath of her suicide. This motif serves as the primary dilemma of the novel, making the novel an attempt to discuss human suffering.
