Disclaimer: I didn't finish this book, so I'm not rating it, but since there don't seem to be a synopsis or any reviews I'll just outline the plot.
I'll also mention that the author's name is misspelled here, it is actually Joan Phipson. If someone with librarian status reads this, perhaps he or she could fix that. Here's the cover, also.
While taking their younger brother to look for ducklings, Hubert and Emily briefly sight a pale, thin girl in a rowboat. When she sees them, she quickly rows away. Returning the following day to look for little Paul's lost glasses, they see the girl again and ask if she needs help. She seems frightened and again rushes off to hide on small island.
In the alternate narrative, the teens' father, a doctor, is brought an hysterical woman who was found on the road by some farmers. Although she is unable to explain what the matter is and no one seems to try too hard to figure it out, the reader is quickly clued in that this is the mysterious girl Bianca's mother and that a series of traumatic events have left them psychologically unbalanced and separated.