The Gossamer Cord — Philippa Carr

When happy-go-lucky Dorabella Denver marries Dermot Tregarland, a charismatic young Cornishman, her twin sister, Violetta, is haunted by premonitions of disaster. Her worries soon multiply, as rumors of blood feuds and mysterious deaths are linked to her sister's new family. With the arrival of Dorabella's first child, all seems to be well, until she disappears under suspicious circumstances. Believing her sister drowned and swept out to sea, a devastated Violetta agrees to live the Tregarlands on their prosperous estate and care for Tristan, her sister's son. But tragedy soon strikes again, claiming another life, before Violetta unmasks the insanity that has plagued the Tregarland family for years.