The Secret Book of FLora Lea by Patti Henry

 Flora and Hazel live with their mum in London as World War 2 is ramping up. To keep them safe, their mother sends them to Oxfordshire, where they are taken in by the kindly Bridie and her son Harry. To keep Flora from worrying about the war and their mother, Hazel makes up a magical land called Whisperwood. One day, when Hazel is distracted, Flora vanishes. Everyone assumes she drowned in the river, but Hazel thinks she is still alive somewhere. Flash forward 20 years. Hazel is working at a rare bookshop and a package comes in containing a book about Whisperwood. Could this be a clue to finding her still missing sister, Flora Lea?  Charming story, good characters.