Then She Was Gone by Lisa Jewell

Ellie Mack was the perfect daughter. And then she was gone.

Ten years after Ellie's disappearance, her mother, Laurel Mack, is trying to put her life back together when she meets an unexpected charming man in a cafe. Before she knows it, she's meeting Floyd's daughters-and his youngest, Poppy, takes Laurel's breath away.

Because the eerily precocious Poppy is the spitting image of Ellie. 

And now the unanswered questions she's tried so hard to put to rest begin haunting Laurel anew. Where did Ellie go? Did she run away from home, as the police long suspected, or did something more sinister happen?