Harahara Shichau!! Oya ni wa Naisho no Kozukuri Seikatsu: Amahara Izumo-hen

You are a male teacher employed in a school where his younger sister studies. One day, a suspicious letter is on your desk. The content is about a threat... a threat to expose the evidence that you are actually sniffing your younger sister's panties behind her back. Guilty, you have no choice but to obey and meet the sender in the drama club room.

(Source: VNDB)