義妹に婚約者を奪われたので、好きに生きようと思います。

"My fiancé was stolen by my stepsister—... Fine then, I’ll live freely! Now that the engagement has been annulled, I was planning to devote myself to my beloved magic tool research and live as I please... but something’s strange about my selfish stepsister!? Light novels? Smartphones? Idol fandom?? She’s saying unfamiliar words, the hostility she used to have has completely vanished, and for some reason, she’s become kind. Could it be that someone has switched places with her on the inside!?"