Broken Princess by K.L. Donn

Aria Adair is the princess of the Empire. Her father—King—rules with an iron fist, and her mother is just as deadly. The one thing Aria has always known, thankfully, is that she’s loved. Her whole life, she’s felt it, reveled in the knowledge that no one could take it away from her.
Until that fateful night when everything changes.
Kidnapped. Beaten. Left to burn alive in the trunk of a car. She has no hope of surviving.

Seven and Severo Petrov–Corelli have always known, from the day Aria Adair turned fifteen, that she would be theirs. They protected her without anyone being the wiser and saved her on the day she was slated to die. Finding her burned, scared, and broken, they sought out her tormentor and exacted the justice Aria deserves.
As she’s healed, picked up the pieces of her teenage life, and learned to live with her scars, the Petrov–Corelli twins have kept their distance. Biding their time and waiting for Aria to reach eighteen so they can lay their claim once and for all.
The trouble with waiting is that it may give someone else a chance to foster the belief that they can stake claim to the Adair princess too. Only this time, she might not survive.