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。

by Hirofumi Neda

When Makoto was summoned to a magical world, he finally found his little sister Misaki, who disappeared two years ago, is now reigning as the Demon Lord of her own kingdom! The bratty Misaki might treat him like a dog, but Makoto knows that the tyrannical Demon Lord is also a lost little girl.