The Last Anniversary by Liane Moriarty

Over seventy years have passed since sisters Rose and Connie Doughty found an abandoned baby near their home on the island Scribbly Gum. With no one to take care of it Rose and Connie took the baby in as their own. Since then the Munro Baby Mystery has brought fame and fortune putting Scribbly Gum on the tourist map. Paperback softback