Greed and Magic (Spells and Sins Book 4) by Melody Raven

They thought the evil was gone, but the true danger is closer than they ever knew... Now that Sam and Derek's relationship is in the open, everything should go smoothly. But New York City hasn't gotten that message. As the families arrive, they leave a trail of bodies behind them, leaving Detective Derek Pierce and his unofficial witch partner (but official girlfriend), Samantha Harris, to deal with by hiding the bodies and finding the killer. Except seeing Derek compromise all of his morals to protect her family makes Sam see that no matter how happy he says he is, she might be the worst thing that ever happened to him. If she wanted to do the right thing, she'd let Derek go. But after everything that's happened to them, doesn't Sam deserve the chance to be a little wicked?