The Full Moon (Under the Moon Book 1) by David Neth

Kathy and her sister, Samantha, have always been a team. Throughout their time as witches, they’ve taken out more than their share of bad guys. But after Kathy meets Will, who she learns is a demonic Dark Knight, her loyalties begin to change. Meanwhile, Samantha doesn’t trust Will or his intentions. Still, Kathy can’t help but feel tempted by the dark side as she falls deeper in love with Will. Crossing over would give Kathy the freedom to do whatever she wanted with her magic. No rules. No limitations. It would also mean breaking the bond she has always shared with her sister, who has made it clear that she wants nothing to do with the dark side. When Will proposes they take over the underworld, Kathy loves the idea of having power. But it also leaves her with a choice that will change her life: abandon her family and the life she has always known, or give up the love of her life forever.