The Lost and the Found. From a Zoella Book Club 2017 author

Real, compulsive and intense: Cat Clarke is the queen of emotional suspense. For fans of Paula Hawkins, Gillian Flynn, Megan Abbott and Jandy Nelson. SHE WAS LOST... When six-year-old Laurel Logan was abducted, the only witness was her younger sister, Faith. Faith's childhood was dominated by Laurel's disappearance - from her parents' broken marriage and the constant media attention to dealing with so-called friends who only ever wanted to talk about her sister.
NOW SHE IS FOUND... Thirteen years later, a young woman is found in the garden of the Logans' old house, disorientated and clutching the teddy bear Laurel was last seen with. Laurel is home at last, safe and sound. Faith always dreamed of getting her sister back, without ever truly believing it would happen. But a disturbing series of events leaves Faith increasingly isolated and paranoid, and before long she begins to wonder if everything that's lost can be found again...