Queen of Darkness

Risty is the youngest princess of the royal family in the demon world, however it turns out that her brother Garum (who she isn’t related to by blood), is going to be the next demon king! Having betrayed Risty before, Risty just can’t imagine the things he can and would do to her if he is to be the next king. And so not wanting to have to deal with him, Risty leaves the demon kingdom and into the human world to establish her own kingdom, and make herself the Queen of her kingdom.

[From White Butterfly's review]