What is the theme in Lock and Key by Sarah Dessen?

Abandonment and abuse go hand in hand as themes of this book. Cora and Ruby are abused by their mother though Cora gets the majority of the physical abuse while the girls are both still at home. The moment she can, Cora leaves the family and the communication is soon brought to a complete halt by their mother. The girls should have known their mother would interfere with their continued relationship but don't seem to recognize that. Each believes her stories about the other. Ruby believes that Cora cut communication and didn't try to keep in touch. Cora believes that Ruby had declined an invitation to Cora's wedding.