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 ever...

Source: MyAnimeList