Secret no Mukougawa

A boy named Yuto has long searched for his missing sister, who disappeared after vanishing through an unusual portal. Seven years later, he witnesses two girls emerging from the same gateway and decides to follow them. He enters the portal and arrives in a perilous, unfamiliar world. There, he locates his sister, but a serious error on his part leads to the deaths of many people in that world. Yuto must now find a way to make amends for his actions and bring his sister back with him.