I think this is easier to read once you understand the MC is not the one actually in charge, the demon is. She's the one enabling his revenge and is she who dictates the course.
I'm saying this because when she was brainwashing that guy in jail to make him believe he kidnapped the girls, she said something about losing her powers. She's most likely using the MC to restore her powers while "helping" him to get his revenge.
The way I see, her priority is to create and expand a harem to absorb their lust or something and restore her power. The sister and the childhood friend will most likely join too.