~!Nayuta is one of 4 other characters to know about Mahiro's gender bending, not including Mahiro herself, the other 3 being Mihari, Mahiro's mother and Chitose.!~

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)
 
Note: Includes 57 extra chapters.

An anthology comic of Onii-chan wa Oshimai! by various mangaka.

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)

Includes: 
1) まひろちゃんお誕生日会場 (2022 March 6) 
2) みはりちゃんお誕生日会場 (2022 April 22) 
3) あさひお誕生日会場 (2022 May 5) 
4) みよちゃんお誕生日会場 (2022 June 18) 
5) かえでちゃんお誕生日会場 (2022 September 12) 
6) もみじお誕生日会場 (2022 October 20) 
7) なゆたんお誕生日会場 (2022 November 23) 
8) まひろちゃんお誕生日会場 2023 (2023 March 6) 
9) みはりちゃんお誕生日会場2023 (2023 April 22)

A booklet bundled with the Onii-chan wa Oshimai! volume 7 special edition.

Nayuta Tenkawa