Angel Factory by Terence Blacker
Twelve-year-old Thomas Wisdom's seemingly perfect life with his peaceful British family begins to fall apart after he learns that he was adopted and that his parents and sister are really angels sent to Earth to prevent humanity from destroying itself. Their plan is to recruit adopted children and persuade them to cooperate with the Project by using brainwashing, threats, and even violence. When he discovers that even the President of the United States is an angel, Thomas doesn't know whom to trust. Then he is forced to make a decision–whether or not to go along with the Project. He discovers that nothing is like it seems and has to decide who to trust. A very engaging YA novel.