Oniichan wa Oshimai!

お兄ちゃんはおしまい！ • Onimai: I'm Now Your Sister! • Publishing

Synopsis

Mahiro Oyama was just a normal erotic-game loving dude...until he woke up one morning as a woman! Turns out his mad-scientist little sister, Mihari, tried out one of her new experiments on him... with a disastrous outcome, as far as Mahiro's concerned! But Mihari is as determined to study him as he is determined to go back to his shut-in, game-playing life, and one thing's for sure... life is going to get a whole lot weirder from here on out! (Source: Kodansha USA)

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