In this amusing piece on book “acknowledgments,” New York Times writer Jennifer Senior includes this bit of real wisdom:
Family members often make the best editors, because they can speak most bluntly to authors, and they have the next-highest stakes. Guess who has to live with you if your book gets savaged?
An editor needs to protect his or her author from ignominy.
basil.CA 
…or if your book becomes a bestseller.