The Code by Peter Lerangis
Twins Evie and Andrew live with their father who is in the military, so they move around a lot. There mother has been missing since their eleventh birthday. She left them a coded note, but they don't know what it says.
When they move to Shoreport, they think that it will just be like any other town they have lived in, but then they receive a package from someone with random-seeming items in it. Neighbors and friends may not be who they seem as Evie and Andrew struggle to find out what happened to their mom.