The Greystone Secrets Trilogy Series by Margaret Peterson Haddix

The Greystone Secrets Trilogy by Margaret Peterson Haddix is a middle-grade mystery/fantasy series involving parallel worlds, secret codes, and family.

• Book 1: The Strangers
  Three siblings discover that kids with their exact names and birthdays have been kidnapped—then their mom disappears, leading them to a parallel world.
• Book 2: The Deceivers
  The Greystones and their friend Natalie return to the alternate world to rescue their mother and uncover secrets behind the strange government.
• Book 3: The Messengers
  The siblings receive urgent messages from the other world. They must act fast to stop a mind-controlling enemy threatening both realities.