If The Crown Fits by Leané Giliomee

A thief has three rules:
#1: Never get caught
#2: Never trust anyone
#3: Never, ever, forget to guard your heart

In the kingdom of Everness, the memory of magic only lives on through whispered stories.
Lara, an elusive bandit raised in a clan of thieves, makes the mistake of stealing from the tyrannical royal family . . . and getting caught.
She has no idea how high the price for her freedom will be.
Forced to choose between forfeiting her life or offering her services to the prince regent, Lara is blackmailed into retrieving a stolen heirloom from Cai, the infuriatingly charming crown prince of the enemy kingdom. Lara must pose as his betrothed — a princess of Everness he is yet to set eyes on.

Bound by a deadly secret and driven by a desperate need to protect those she loves most, Lara’s path to freedom is filled with deceit and desire. But it can only lead her to becoming an enemy of the crown, or an enemy of her own heart.