Onna Kishi Juku ni Nyuugaku shitara, Otouto Skill de Mugen Amayakasare!?

Otool, who bears a striking resemblance to his super-genius older sister, is forced to enroll in the Miife Girls' Knight Academy in her place after she suddenly disappears. Disguising himself as a girl to avoid being discovered, he quickly finds himself embroiled in trouble. "If only I had the powers of my sister... Help me, sis!" Otool cries out, only to awaken his 'Absolute Little Brother Ability'!? A dreamlike fantasy in another world begins, where he is spoiled rotten by the older sisters of the academy!

(Source: Overlap, translated)