My Sister's Keeper, Jodi Picoult

In all thirteen years of Anna's life, her parents have never given her a choice: she was born to be her sister Kate's bone marrow donor and she has always given Kate everything she needs. But when Anna is told Kate needs a new kidney, she begins to question how much she should be prepared to do to save the older sibling she has always been defined by. So Anna makes a decision that will change their family forever - perhaps even fatally for the sister she loves.