What is the main conflict in Lock and Key by Sarah Dessen?

When Ruby is five, her father leaves and her mother, suffering from alcoholism, a lack of self-esteem, and abandonment issues, begins a series of dead-end jobs, erratic moves, and abuse. Ruby's sister Cora stands between Ruby and her mother, literally, for the next two years until Cora turns eighteen and goes away to college. She initially calls but the girls soon lose touch.