The Flame Never Dies by Rachel Vincent
Nina Kane was born to be an exorcist. And since uncovering the horrifying truth-that the war against demons is far from over-seventeen-year-old Nina and her pregnant sister, Mellie, have been on the run, incinerating the remains of the demon horde as they go.
In the badlands, Nina, Mellie, and Finn, the fugitive and rogue exorcist who saved her life, find allies in a group of freedom fighters. They also face a new threat: Pandemonia, a city full of demons. But this fresh new hell is the least of Nina's worries. The well of souls ran dry more than a century ago, drained by the demons secretly living among humans, and without a donor soul, Mellie's child will die within hours of its birth.
Nina isn't about to let that happen . . . even if it means she has to make the ultimate sacrifice.