What is the theme in Sister: A Novel by Rosamund Lupton?

An important theme in this book is grief. Grief is felt from the moment the book begins. The reader senses that something has happened to Bee's younger sister Tess, and it is not until a few chapters in that it is confirmed that she has died. Once this is confirmed, the grief builds in the many of the characters, for Tess was greatly loved. Soon it is learned that Tess was the first to suffer from grief.