The Pretty One by Clare Boyd

Nell is the perfect child; more beautiful and more likeable and more loveable than me. If my pretty little sister never came home again, I'd finally be free from the terrible secret we share.
Anna is a wonderful mother. She gave up a career she loved to stay home with her children and her kind, polite daughters, Nell and Ivy, are a credit to her. As is Bay, her stepdaughter, whom Anna adores as much as her own two girls. But everything changes the day Anna's middle daughter's beauty catches the eye of a stranger in a café.
Quiet, sensitive Nell doesn't want to have her face on a billboard for all to see. But she knows her family desperately needs the money, and perhaps it would be nice to have her mother's full attention for the first time since Bay arrived? Could this be the escape from her stepsister's twisted games Nell has been longing for?
But the moment Nell steps in front of a camera, the devastating secret she and Bay have been keeping breaks the surface – and the bonds of this close-knit family forever. But with both Nell and Bay insisting they're innocent, can either girl really be trusted?
As Anna's perfect blended family unravels around her, she is forced to pick a side. To save one daughter, Anna must betray another. Choosing the wrong girl will be more dangerous than anyone could ever have imagined.