Isekai Ittara, Sudeni Imouto ga Maou to Shite Kunrin Shite Ita Hanashi.
異世界行ったら、すでに妹が魔王として君臨していた話。
Makoto is transported to a fantasy world where he discovers his missing younger sister, Misaki, has become the ruler of a kingdom as the Demon Lord. Though she acts spoiled and domineering, Makoto understands that beneath her harsh exterior is a frightened and lonely child.