The Sinner and the Gunslinger by Lana Pecherczyk

The age of sin is here…

Chased by evil her whole life, Leila was accused of lighting the fire that killed her beloved foster brother Zeke. Now she’s one of the badass assassins fated to wield holy relics that will stop the apocalypse. But when Zeke turns up alive, handsome as the devil and working for her rivals, she’s forced to confront her worst fear… is she evil? Why else would he fake his death and abandon her?
Burdened by guilt, Zeke joined Team Saint as a gunslinging demon hunter to pursue a path of redemption. He failed everyone in his life… including the only girl he ever loved, his foster sister Leila. When an ancient prophecy drops him back into her life, he realizes he has a second chance at making things right. Easier said than done when she would rather he stayed dead.
As Lilith’s dark plan unfolds, Zeke and Leila are forced together on a desperate quest for a weapon that will stop the Horseman of War. Past wounds reopen, sins are confronted, trust is shattered, and passions ignite. And when lies are circling like sharks, their newly forged love could be the salvation the world needs… or the downfall of them all.