Isekai One Turn Kill Neesan: Ane Douhan no Isekai Seikatsu Hajimemashita

Asahi Ikusaba is transported to another world after trying to save a child and initially assumes he has finally gained the overwhelming powers given to heroes in his favorite stories. His first serious monster encounter destroys that fantasy: Asahi is still painfully ordinary. When he cries out for his older sister Maya, she actually appears, and she is the one who has arrived with enough power to defeat his enemies in a single blow.

Maya happily decides to protect her beloved little brother from everything, while Asahi tries to look like an independent adventurer despite the fact that his strongest weapon is his sister. Between monsters, quests, and constant misunderstandings, the siblings begin a new life in which they rarely agree on who is supposed to be protecting whom.