Sleep Sister / Laura Elliot.

Enhanced descriptions from Syndetics:

Two childhoods destroyed. One story they will never tell. Until now. Beth ran away from her family when she was a teenager. She left behind a terrible evil that took her innocence. She also left behind her sister, Sara. When Beth returns home, she is shocked to discover her terrible secret is not just hers alone...she shares it with Sara. Under the shadow of a remote headland, the sisters make an oath they promise never to break. Eva's birth is a mystery that remains unsolved. Years later with her marriage in ruins, and her future uncertain, she realizes that to move forward with her life, she must first understand her past. But while Eva is drawing closer to the truth about her roots, Beth and Sara's lives are falling apart, crushed under the weight of the secret they carry. They must confront the past and face the darkness once more. But this time, their story will be heard. From the bestselling author of The Betrayal, Stolen Child and Fragile Lies, comes a breathtakingly tense and emotional story of the fierce bond between sisters, and a family destroyed by a disturbing secret. Note: Previously published as 'When the Bough Breaks' Read what everyone is saying about Laura Elliot's books: 'My emotions were in overdrive...absolutely gripping - one of my best reads of the year so far.' Literary Lightbox 'Entertaining and highly thought-provoking.' Closer Magazine 'This page-turner is gripping, all the more because it presents the dilemmas of betrayal with brutal honesty.' Irish Independent 'The Betrayal certainly makes for a tense read that had me at times literally on the edge of my seat.' By The Letter Book Reviews 'This well-crafted and compelling story traces the deceits which begin unnoticed but end in the destruction of friendships and lives.' The Irish Times