Chiyodaku Oukoku Judgement: Ane to Ore to de Isekai Saikou Saibansho

Akuto lives a normal life, often dreaming of traveling to fantasy worlds and having adventures. To his surprise, his dream comes true, but this other world isn't quite what he expected. The magical land of Chiyodaku doesn't need a champion—it needs someone to oversee court trials. Fortunately, his older sister, Tsukasa, happens to be Japan's greatest judge. And although she knows nothing about sorcery and monsters, that's Akuto's area of expertise as a game-obsessed high schooler. Together, they're the perfect team to bring justice to the kingdom—at least they'd better be, because one of their fifirst cases is a murder trial...and the Hero is the accused!

(Source: Yen Press)