The Kiss By James Patterson

Whit and Wisty Allgood, a witch and wizard with extraordinary abilities, have defeated the ruthless dictator who long overshadowed their world. But for the first time in their lives, the powerful brother and sister find themselves at odds as Wisty is drawn to a mysterious and magical stranger named Heath.
Wisty has never felt as free as she does with Heath, especially when the two of them share and test their magic together. But when a merciless Wizard King from the mountains suddenly threatens war, Wisty must make an excruciating choice. Will she unite with Whit to fight the mounting dangers that could return their world to a tyrant's domain? Or will she trust the beautiful boy who has captured her heart?
James Patterson's epic dystopian saga continues as the witch and wizard who have inspired countless imaginations must rally together before the world they fought to save collapses.
 
- Goodreads Summary