In the Forests of the Night by Amelia Atwater-Rhodes

Risika is a vampire who was changed by another named Ather, in front of her twin brother and Aubrey, whom she believes killed her brother. She stalks Aubrey in a deliberate game of cat and mouse while determining whether she has the strength to defeat him.

During one of their battles, however, Risika discovers her brother wasn't killed that night--he was already a vampire. What should she do now, when the reason for her revenge has disappeared?

Short and action-packed, I enjoyed reading about Risika and her fight for justice.