Among The Barons by Margaret Peterson Haddix

A hidden third child who has lived his life in secrecy is sent to live among a powerful, wealthy family and must pose as one of them; as he experiences privilege, discovers the corruption and political maneuvering that supports the Population Law, and forms risky friendships, he faces difficult choices about trust, sacrifice, and whether to take actions that could expose him and help other shadow children.

Purchase from Bookshop.org