At the start, Julia is the vehicle for revealing Sarah's tragic tale, for as she learns more about what happened in Paris during WWII, the reader is fully invested in Sarah and her family. Sarah is the compelling character, her story line is moving and very well done. That is, until the conclusion of her character arc, which is drawn out unnecessarily by the author, through Julia's character.
The book should have concluded with Julia wrapping up Sarah's story, rather than dragging out Julia's revelations about herself and her family's situation, which was not only dull, but emotionally manipulative. Julia is not nearly the sympathetic character that the author thinks she is, and by dragging the reader through her life, the book loses its focus and its impact.