Tethered Souls by Miranda Grant

Someone Varius trusts wants him dead.
And so do I.
They call him an abomination, a freak of nature just because he's a witch without magic. My hatred of him goes deeper than that. He's a monster wanting to marry my baby sister, to kill her hopes and dreams by turning her into nothing more than a womb to push out his heirs.
Well, I'm not going to let that happen. I sold my life to save hers fifteen years ago, becoming an assassin for our father to use for his darkest of sins. Now I have a special skillset and a name I'm willing to kill for free.
But what I don't know is Varius never planned to marry her. No, the woman he buys in my father's office...
...is me.