Robi's Flying Saucer Drive-In - Robi's Flying Saucer Series, #1

Away from the brooding home of her family, Saffron can work at Robi's Flying Saucer Drive-In restaurant and become part of a new family. When Clair arrives with the carnival that sets up out back, Saffron has a new friend. The carnival disappears after a cataclysmic storm and Clair is left behind. Clair comes to live in Saffron's basement. There is a lookout for Clair, but Saffron fears that this girl, who looks fifteen but is truly centuries old, doesn't feel danger.
Saffron's small sister Lily, and somewhat friend, Fred, become part of a group that keeps Clair safe and discovers her truth.