Mystery of the Hidden Hand — Phyllis A. Whitney

As soon as Gail Tyler and her brother Warren come to visit their mother's family on the Grecian island of Rhodes, they are caught in the tangles of a dark family secret. Gail is soon enmeshed in a series of events that pull her deeper and deeper into mystery. The unraveling of the secret begins at the white house, leads to the ruins of Camiros, and reaches an exciting end on the rock of Lindos, high above the Aegean. Here is an absorbing story of modern Greeks, historical Greece, and one girl's discovery of her own special worth.