Onimai: I'm Now Your Sister!

Mahiro Oyama is a self-proclaimed home security guard who hasn't left his house in years, spending most of his time playing erotic visual novels. His sister Mihari, who's concerned about his well-being, devises a plan to help him become more normal. As part of this plan, Mihari creates a medicine that changes Mahiro's biological sex to female, which he strongly resists. Now living as a girl, Mahiro has to navigate this unusual situation until the effects of the medicine wear off.