Tilea no Nayamigoto: Isekai Jashin Tensei Kitan

Summary

With the progression of his Chuuni Disease, Shioda Tetsuya is shunned by his family, and dies in despair, a thirty three year old jobless virgin. He vowed that if he was born again, this time he would stay away from delusions and live life seriously. Fortunately, he reincarnated in another world as a girl named Tilea, and lived a happy life with her kind parents and adorable little sister, but… Lately, she'd been troubled. Her beloved sister had begun talking about how she was actually one of the Six Demon Generals, serving directly under Demon King Zorg, a descendant of the Three Great Demons. The situation was looking bad. She had to do something before this Chuuni Disease got any worse! This is a fantasy "misunderstanding story" about a protagonist who has not noticed in the slightest that she's a "cheat"-like existence, and how in her efforts to heal her sister's Chuuni Disease (except that she really is one of the Six Demon Generals), she somehow or other ended up setting out to defeat the Demon King (Source: Briollaire Blog)