Her brother wants her because he is obsessed with her. He ends up getting her too, but deep down she knows she hates him (that is after she was brainwashed) and he has her watching doinking other woman to try and make her jealous
I'm warning you before reading this spoiler, her brother is beyond fuc*** up
One more thing: I have mentioned in another spoiler that they aren't actually siblings, well that is not the brothers real appearance too. The empress is a mage and she used magic to make him look like he is of royal descent when he is not. That is also how he became King.
