[Ilparazzo]

Originally Posted by Archanfel

Thanks for the info and the CG.

Found the short description of the game by Honoohhh.
Game Synopsis:
The main character lives with three younger sisters. They don't care about him, so they often walk around the house without clothes or come into bathroom while he has a bath. He is confused by their demeanors and he starts having a dream of a girl who taunts him without notice. One day, the girl in his dream suddenly appears. According to her, she is a princess of the devil world and he is a prince. This means he is the one who will take over the throne. As a matter of fact, his three sisters are descendants of a magician and they all know it. There is only one way to stop it. If one of three sisters gets pregnant with his baby, he won't need to take over the throne. The sisters soon curse him not to be able to ejaculate without them. A fierce battle for him now breaks out like this....

From the description I doesn't seem to contain evil conversion

Sounds like it, unless you count magician turning into a devil girl as conversion.