Sister

Many older sisters take their "job" seriously, and in Rosamund Lupton's Sister, Beatrice is no exception. Despite moving from London to New York, she stays close to her sister Tess, speaking and emailing frequently.  When Bee's mother calls to let her know Tess has been missing for several days, Bee gets the first flight home, planning to locate, and lecture, her sister.  However, she never sees her sister alive again.  Tess's body is discovered in an abandoned park restroom, and the police rule the death a suicide.  Bee knows her sister would never take her own life, but can't convince anyone else of the fact, so she takes the investigation into her own hands. Is Tess's murderer her married school tutor with whom she had an affair resulting in pregnancy? Tess's classmate and stalker? The more Bee investigates, the more possible suspects she encounters. If she can't find someone to believe her, Bee's life might also be in danger.  

Written as a letter from Bee to Tess explaining what had happened, this was a quick read that I didn't want to put down. The twist at the end was surprising and unexpected, but I felt a bit unsatisfied.