Onimai: I'M Now Your Sister!

Mahiro Oyama was an ordinary guy who had an interest in erotic games. However, his life took a sudden turn when he woke up as a female one morning. His younger sister Mihari had used him in one of her scientific experiments, which had a catastrophic effect on Mahiro. Despite his desire to return to his previous lifestyle of playing games and remaining indoors, Mihari was determined to study him, and things were bound to get even more bizarre from here on out.