What is the main conflict in Housekeeping by Marilynne Robinson?

The novel's main conflict revolves around family and isolation. Ruth and Lucille are isolated from everyone around them. First their father's abandonment then their mother's depression and suicide set them adrift. Their grandmother takes care of them, but she is elderly and she also loses all of her loved ones. Her husband dies in the train derailment, her daughters all leave home, and Helen comes home, not to see her mother, but to drop off her children and commit suicide. Their grandmother seems distracted, as if not able to really see Ruth and Lucille.