PA: The Missing - Found (book 1) by Margaret Peterson Haddix

               The Missing: Found (book 1)                                   by: Margaret Peterson Haddix

The protagonists in the book Found would be Jonah Katherine and Chip.
Jonah is a 13 year old boy, he is adopted and lives with his adoptive parents and sister Kathrine. Chip is his best friend also 13, he just found out he is adopted and doesn't know how to deal with it. Katherine is Jonah's younger sister who is 12, she is not adopted, but wants to help Jonah and Chip learn about their past. They receive strange mail telling them to BEWARE and SOMEONE IS OUT TO GET THEM and THEY ARE COMING BACK FOR YOU, They decide to figure out who they really are, finding out they are in danger, they have time traveled from where they really are from and someone wants to come and send them away forever.
The antagonists are Gary and Hodge they are a pair of kidnappers, very big and strong, from the original time, not time travelers, hired to kidnap the 36 missing kids and send them back in time to where they were originally supposed to be (the 36 missing kids in the past) and then telling their parents that they all died and have no idea what is going on.