I thought the above commentary was unduly harsh. To be fair I have the book in my more recent memory, but I remember the story dealing with a very complex set of ideas in a way that children could understand. [Granting the the movie was only billed as being family friendly, while the book was expressly aimed for young adults.] It wasn't supposed to be a surprise whether or not this woman was really their older half sister. The important thing was the realization that it really did not matter. A strange woman comes to a wounded family and presents herself as a part of it. She then proceeds to fix many of the problems that plague it- she literally saves the children. The boy is rescued from isolation and the girl from a life somewhere between a pet and an embarrassment.
There are some things I might be filling in from the book, but I don't remember finding the movie lacking. I was even motivated to keep an eye out for Father's Archane Daughter to read it. Some minor changes or things left unsaid were inevitable- Caroline disappearing in a plane crash instead of a kidnapping, the younger children's mother's emotional distance, the full role of of Caroline's old headmistress- etc. The only thing I can think of that was wholely snipped was the interesting relationship between the father and Caroline. They probably thought they could not do it justice so they thought it better to leave it out. Plenty of movies have suffered from the unwillingness to make that sort of decision.
On the whole I recommend the movie, and I truly recommend the book (along with everything else if E. L Konigsberg I have gotten around to reading- I think she just collects Newberys)
