The Distant Hours: Kate Morton

Edie Rurchill and her mother have never been close, hut when a j-lost letter arrives one Sunday afternoon with the return address of Milderhurst Castle, Kent, printed on its envelope, Edie begins to suspect that her mother's emotional distance masks an old secret.
Evacuated from London as a thirteen-year-old girl, Edie's mother was chosen by the mysterious Juniper Blythe and taken to live at Milderhurst Castle with the Blythe family.
Fifty years later Edie, too, is drawn to the castle and the eccentric Sisters Blythe. Old ladies now, the three still live together, the twins nursing Juniper, whose abandonment by her fiance in 1941 plunged her into madness.
Inside the decaying castle Edie begins to unravel her mother's past. But there are other secrets hidden in the stones of Milderhurst Castle, and Edie is about to learn more than she expected. The truth of what happened in the distant hours has been waiting a long time for someone to find it.