Imouto ga Onna Kishi Gakuen ni Nyuugaku Shitara Nazeka Kyuukoku no Eiyuu ni Narimashita. Boku ga.

My sister enrolled in an all-girls knight training academy. Even at the kingdom's most prestigious academy, my little sister, a brilliant girl, excels in every subject... "I'm telling you that my brother is so much stronger than me!" No, I'm just an ordinary guy.......? (*That's not true. He just doesn't realize it yet.) Because of that boast, the top knight of the school challenges me to a mock battle. (*The strongest knight of the school realized their difference in strength in an instant.) Now make sure you cool down properly after exercise! (*My brother's massage made the lady knight so happy that even other nobles were impressed!) Just doing my normal thing, I somehow caught the attention of the princess? A harem of lady knights for a self-proclaimed ordinary guy who doesn't recognize his own power! (Source: Comic Alive, translated)