Brocon no Ane ni Jitsu wa Saikyou Mahoushi da to Bareta. Mou Gakuen de Jitsuryoku wo Kakusenai

Alvin, possessing top-tier abilities, wants nothing more than an easy life, so he's been pretending to be incompetent. However, his doting older sister (who even proposed marriage to him!) accidentally witnesses his true power! "I knew it! I believed in you, little brother! I have to go tell everyone right away!" The rumor spreads like wildfire, and Alvin's treatment at the academy completely changes! He instantly defeats students who dare to challenge him, is recognized as the strongest mage by the princess (who also happens to be the knight commander), unexpectedly rises through the ranks of the knight order, and somehow finds himself surrounded by more and more girls—where did all the peace and quiet go!? "You're so cool! I think I’m falling even more in love with you, little brother!" ...Meanwhile, completely oblivious to Alvin's anxieties, his sister gives him a huge hug.