Into The Wind The Northwind Chronicles Book 1 by Karen Aileen Saunders

Eight year old Will and his sister, eleven year old Sara lived in the largest mansion in the United States in 1952, owned by their late grandfather. It became their prison until the fateful night of his death, forcing them to escape the treacherous plans of one to kill them for their massive inherited fortune. Surviving a shipwreck, sharks and a women that would not stop till they were dead, Will and Sara become free at last.