The Hawkweed by Candace J Thomas

Jaren Iniav doesn’t know how to live after the death of his twin brother, Spen. The government claims his death as an accident, but Jaren knows better. Faced with no choice, Jaren joins the Tundran army, set to uncover the truth, unprepared for what he discovers: a secret operation of programmed assassins sent to kill the woman he left behind.