The Truth According to Us, by Annie Barrows

Willa (12) and Bird (9) Romeyn live with their Aunt Jottie (and sometimes their father, Felix) in Macedonia, West Virginia.  Exiled from her pampered life, socialite Layla Beck takes a room in their home, when she begins a job with the Federal Writers' Project writing a history of mill town Macedonia in time for its 150th anniversary.  In tracing the history of the town, Layla meets a bevy of entertaining characters and uncovers the secrets behind the fall of the once influential Romeyn family.