The Catalyst by Josephine Bell

In this modern-day Greek tragedy, the Fates play havoc with a middle-aged English trio who have planned a restful and much-needed holiday in Greece. Hugh Wilmot, his wife Florence, and her sister Beatrice have secretly loathed one another for years. Confronted with the pressures of new people and new places, their emotions rise to the boiling point under the hot Aegean sun.