The Secret in the Sand Castle — Laura Lee Hope

The moment Bert and Nan, Freddie and Flossie arrive at the old Wilson house, they have a feeling their vacation in Beachcliff Bay is going to be unusual. And they're right! Before the day is over, they're wrapped up in a mystery. Why does a mirror break when no one is around? Is the Wilson house haunted? Most important, what happened to the treasure that the last of the Wilsons hid -- just before they disappeared at sea?