Obsessively Yours by Jamie Applegate Hunter

Roman
At twelve years old, Roman Covington—crown prince of the tropical fae kingdom—met Violet Maekin: a girl with freckles on her nose, a razor-edged wit, and the ability to make him laugh. She made him feel normal in a world that demanded he be anything but. After just a single day and a few scribbled notes, she held his heart in her hands.
But when Roman turned thirteen, the gods granted him the name of his fated mate—Vivian. Violet’s identical twin sister.
Convinced the gods were cruel, mistaken, or both, Roman spent a decade seeking a way to shift the bond from one sister to the other. All the while, a quiet madness bloomed within him.
He watched Violet from the shadows.
Stalked her in silence.
Killed for her in the dark.

Violet
When the king and queen arrived to announce the prince’s gods-blessed mate, Violet already knew it was her.
Except it wasn’t.
Vivian got Roman, and Violet got nothing but a splintered heart and the cruel ache of pretending it didn’t break her.
But Violet and Roman’s friendship endured… until the day she realized her feelings had grown into something vast and soul-deep, far beyond the reaches of her childhood crush.
Roman and Vivian might avoid one another now, but Violet knew the gods did not make mistakes. Their indifference would turn to love while Violet’s heart would turn to ash.
She did the only thing she could to save her heart and walked away from Roman for good.