The Missing Magician - Carol Fisher Saller

Ozzie is the only cousin Ellie and Kat have ever known, and his guardian, a beloved magician, has literally disappeared. So what if Ozzie's from England in the year 1323? Not a problem when your mom has a time machine. The twins sneak off to medieval Ackenbury and join their cousin's band of traveling musicians and acrobats to follow the clues.

It's good timing: Both sisters are glad to get away from school. Kat is still struggling to make friends, and Ellie has accidentally spread a rumor about a new girl that turned out not to be true.

Juggling secrets, lies, a daring rescue, and some really weird food, the twins learn that family isn't just something you inherit. It's something you'd cross centuries for.