Idaho
The setting of Idaho is an oppressive imagery to the girls, especially when they see what the winters are like. It isn't just snow. Snow means they are captives in their home, because they can't leave, and there's nowhere to go if they could. Winter brings a standstill to life that they are unfamiliar with, and the imagery drives them crazy, because they are forced to sit still and think about the tragedy and trauma they don't want to face.
Chaos and order
Aunt Sylvie's home is not a healthy environment for the girls, because they are so full of chaos and pain that all they want is to be in an orderly environment, but instead, they are trapped in a house that is full of disrepair and chaos. When they try to clean, they get into conflicts, and Aunt Sylvie is not a good parent. The courts get involved and see the conditions of the home. The girls decide to go two separate ways.
Death and experience
The imagery they can't escape but hate to face is the imagery of death. They experienced death secondhand, watching their parents pass away, and their normal life is gone forever. In light of that pain, none of their previous opinions even seem to matter. They are full of remorse because they can't un-experience their suffering. They cannot go back to the blissful innocence that came before. They can only go forward.
Normalcy and suffering
The girls take two antithetical stances on the issue of normalcy. One girl decides to be chaotic, accepting that her unusual suffering has made her an unusual person, and the other strives for normalcy. Lucille tries to put it behind her, but inside her there is still all the chaos that Ruthie accepted. The implication is that in Ruthie, there is still a thirst for order, so that both girls can be said to be in the grips of tragedy.
