The Faerie Path: Lamia's Revenge #1: The Serpent Awakes

Tania was living an ordinary life in the Mortal World until her sixteenth birthday, when she discovered that she was the long-lost princess of the Royal House of Faerie, daughter of King Oberon and Queen Titania. As the only Faerie who possesses the ability to walk between the Immortal and Mortal Worlds, she's balancing her two lives as best she can: making time to hang out with both sets of parents, getting to know her new sisters, and spending any spare moments with her boyfriend, Edric, a member of the Faerie court. But now an evil force has come to the Immortal Realm, targeting those Tania holds most dear—and threatening the entire Faerie world.