The Ribs of Death // The Crystal Crow — Joan Aiken

Aulis had been reluctant to recuperate in Cornwall but she soon found herself enchanted by the dreamy seascape and magnetically drawn to Charles Foley, although he had only a year to live. But uneasiness soon gnawed at Aulis' peace of mind. Eleanor, Charles' sister, hated her. Aulis worried about the strange control that Eleanor, a doctor, exercised over their lives. And as she learned more, her fears grew. What tragedies from the doctor's past had turned that beautiful woman into a ruthless psychopath, who used her profession not to heal but to murder?