As in most cases of family members being in an asylum, there are secrets. Deep dark ones that affect Iris and can explain her lack of knowledge of Esme.
The book is told in 3 voices. One is in the present time telling of Iris' story. The other is a narrator telling of episodes from the young life of Esme Lennox. The last is the inner monologue of Iris' mad mother, Esme's sister. The voices are confusing at first but one is finally able to make out some sense and piece together the horrific story of Esme Lennox. Throughout the book, there is a certain amount of uncertainty and suspense, actually. Was Esme a crazy ass old lady who was going to stab Iris to death? Or was she a victim. The reader soon knows, and it is a haunting and sad answer.