The Distant Hours by Kate Morton

A long lost letter arrives in the post and Edie Burchill finds herself on a journey to Milderhurst Castle, a great but moldering old house, where the Blythe spinsters live and where her mother was billeted 50 years before as a 13 year old child during WWII. The elder Blythe sisters are twins and have spent most of their lives looking after the third and youngest sister, Juniper, who hasn’t been the same since her fiance jilted her in 1941.

Inside the decaying castle, Edie begins to unravel her mother’s past.