Archive of Fire [Sentinel Book 1] — Betsy Dornbusch

Twins Aidan and Kaelin didn't realize until they got to university that most guys don't learn five ways to kill a man by the age of fourteen. Still, since their estranged father descends from the demon Asmodai, it's probably worth knowing how to defend themselves. But as years pass and threat never materializes, the twins suppose their mom is just paranoid - until she disappears. Their father tells them Asmodai has taken possession of their mother in order to infiltrate Sentinel, a treacherous coalition of demidemon rebels determined to protect humankind from the demon legions. The twins form a grudging alliance with Sentinel to rescue her, but when Asmodai murders their father to incite war, Sentinel starts to implode and Aidan and Kaelin must battle an enemy who wears their mother's face.