Hard to Kill: a Hard Targets novel (Hard Targets #1)
Wendy Byrne, Gemma Halliday

ABOUT THE HARD TARGETS TRILOGY:
Goren Petrovich was an infamous KOS agent in war-torn Yugoslavia, know for his ability to turn orphaned children into highly trained government assassins. One of his most notable protégées was Anya Danielovich (from the novel Play Nice by Gemma Halliday). But after the KOS disbanded and Petrovich fled to the U.S., he continued his training "academy"—this time for personal profit. Three of his talented U.S. pupils: Sabrina, Jake, and Max Shaw. But when the siblings break from his grasp and attempt to start over, living normal lives that have nothing to do with guns, death, or lies, they learn that you can never really escape your past...
Hard to Kill (book #1), Hard to Trust (book #2), Hard to Stop (book #3)