Philippa Carr is a pen name for Victoria Holt.
From Publishers Weekly
Plodding and predictable, this novel set in pre-WW II Britain sports the classic gothic trappings of a castle wherein stalk madness and murder, but little of the brooding tension needed to make such elements thrilling. Violetta and Dorabella Denver, aristocratic 17-year-old twins, are visiting friends in Germany when they meet a romantic Cornishman named Dermot. Frivolous Dorabella marries Dermot and takes up life in Tregarlands, his somber mansion on the isolated Cornish coast. But it is Violetta, the practical twin and narrator, who learns of the gruesome legend that haunts Tregarlands and discovers that Dermot's pregnant first wife mysteriously drowned. After Dorabella bears a son, disappears and is believed drowned herself, Violetta sets out to solve the riddle of her sister's fate, with a solution quickly becoming obvious to readers.