The Strangers (The Greystone Secrets #1) by Margaret Peterson Haddix

What makes you you?

The Greystone kids thought they knew. Chess has always been the protector over his younger siblings, Emma loves math, and Finn does what Finn does best—acting silly and being adored. They’ve been a happy family, just the three of them and their mom.

But everything changes when reports of three kidnapped children reach the Greystone kids, and they’re shocked by the startling similarities between themselves and these complete strangers. The other kids share their same first and middle names. They’re the same ages. They even have identical birthdays. Who, exactly, are these strangers?

Before Chess, Emma, and Finn can question their mom about it, she takes off on a sudden work trip and leaves them in the care of Ms. Morales and her daughter, Natalie. But puzzling clues left behind lead to complex codes, hidden rooms, and a dangerous secret that will turn their world upside down.