Sister by Rosamund Lupton

Nothing can break the bond between sisters ...When Beatrice gets a frantic call in the middle of Sunday lunch to say that her younger sister, Tess, is missing, she boards the first flight home to London. But as she learns about the circumstances surrounding her sister's disappearance, she is stunned to discover how little she actually knows of her sister's life - and unprepared for the terrifying truths she must now face. The police, Beatrice's fiance and even their mother accept they have lost Tess but Beatrice refuses to give up on her. So she embarks on a dangerous journey to discover the truth, no matter the cost.