Phoenix Rising by Allison A. Andrews (Order of the Dragon #3)

Having finally come into the powers that were foretold, Isolde is now faced with the task of ending the vampire war that has raged for millennia. But the prophecy said that her vampire lover was meant to be at her side, and he is nowhere to be found after discovering her mixed feelings towards his twin brother. The twin brother who turned out to be not quite as evil as everyone has believed for the last five hundred years.

Torn between her duty and her need to make things right with Liam, Isolde must now make some of the toughest choices she will ever have to make. And face down the twin sister she hoped never to see again.

But will she be able to make the final sacrifice and save humanity without losing herself in the process?