The Serpent's Sin: A steamy dark fantasy romance (Bloodlines) by Kathryn Ann Kingsley

To destroy the vampires who killed her family… she must become one of them. Nadi is trapped in a deadly alliance with Raziel Nostrom. After a honeymoon soaked in blood and desire, they make a pact—to take down his criminal family together. Now Nadi is at the heart of the Nostrom empire, pretending to be a vampire. If her act slips even for a moment, they will tear her apart. Raziel holds Nadi’s life in his hands. And her heart… His wicked smile sets her body aflame, drawing her closer and closer. Still, he is a born killer—alluring and utterly ruthless. He could turn on her in an instant. But Raziel is not the only one in his family with a taste for betrayal… When his brother and sister approach her with proposals of their own, Nadi faces a desperate choice. Will she stand by Raziel, as the world turns against him? Or seize the perfect chance to destroy his family forever? Read more