ShadowBane - A Schism of Souls, #1

One womb, one soul, two fates. One will rise. One will fall.  Shura and Jin are the last of a divine bloodline, operating at the bloodiest fringes of society where they wield might and magic against the demons they are sworn to destroy. For Shura, it's the only way to restore honor to her lineage and claim her rightful place in the world. For Jin, it's a matter of begrudging duty-and escaping the influence of the three kings vying for control of the continent. In their pursuit of the sadistic cult behind the demons' resurgence, the twins find themselves hunted by an ancient demon harboring a blood vendetta, and are forced to set aside past grudges and seek the kings' alliance.
When Shura discovers a chilling prophecy, she fears that even her newfound allies may not be enough to stem the rising tide of shadow.  Even the palest chance of victory will come at a brutal price. But the prophecy never foretold whose fate would settle the debt: Shura's or her brother's.