Onii-chan wa Oshimai!

From Kodansha: 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.