A Deal with the Dark Fae by Valerie Harmon

A feisty heroine with a secret. Two brothers, the exact opposite of each other, neither up to any good. Once her secret is revealed, there is no going back.
You can’t say “no” to the Dark Ones. For they are fickle, and you risk paying too high of a price for your refusal.
They can’t be outsmarted.
They don’t accept being wrong.
I knew these rules since childhood. I also knew that it was in my best interest not to get involved with them. But I did.
And now I have to stay close to one of the wretched Horos brothers. The man who’s considered to be the most powerful Dark One in Grassor.
While he’s getting on my nerves, his brother is trying to figure out who I am. But there’s no way I’d let him get into my head or my heart.
Because he’s of the dark, and because… he has a fiancée.