Mihari and Her Onii-Chan

Mihari and Her Onii-Chan (みはりちゃんとお兄ちゃん, Mihari chanto o nīchan) is a bonus chapter of Onimai: I'm Now Your Sister. It was originally published exclusively in Volume 1 of the manga which was released on June 27, 2018 in Japan, and August 31, 2021 in the US.

Summary
Mihari recalls discussing with Kaede in middle school about wanting a younger sister, and in the present day reflects on how much things have changed since she began her experiment with Mahiro.

Plot
A few years ago, when Mihari and Kaede were first-year middle school students, the two say good morning to each other outside of school. Kaede asks about a new hairclip that Mihari is wearing on her middle bang, and Mihari tells her that her older brother gave it to her as a reward for doing well on a test. Mihari remarks that she'd better work hard on the next test as well, and Kaede realizes that Mihari really loves her older brother.

Mihari asks Kaede if she has any siblings, and Kaede mentions that she has a younger sister. Mihari remarks that that sound wonderful and mentions that if she had a younger sister she would spoil her rotten and dress her up in all sorts of outfits. Kaede remarks that it only works like that if the younger sister likes the outfits you pick for her, mentioning how her sister is kind of a tomboy so she can't do that. Kaede does remark though that her sister still is awfully cute and that she bakes cookies and stuff for her sometimes.

Mihari is very excited by the fact that Kaede can cook and asks Kaede to teach her, to which Kaede agrees. Mihari suggests that they do it after school today, much to Kaede's shock. Mihari smiles and thinks to herself that her older brother is going to be so happy.

Back in the present day, Mihari is baking a batch of cookies. She remembers how eventually her kind older brother stopped making time for her, so she instead worked hard on academics and sports. She remarks that now things are different and takes the cookies out of the oven.

She goes into Mahiro's room and announces that she's made some cookies. Mahiro becomes happy and devours the entire tray, telling Mihari how good her cooking is. The sudden compliment causes Mihari to blush.

Mihari thinks to herself that now that her older brother is a girl, they talk a lot more just like old times, even though they've swapped places. She thinks that her project might already be a success.

Just as she thinks this, Mahiro remarks that she's stuffed and decides to take a nap, much to Mihari's shock. She tries to rustle Mahiro awake saying that if she sleeps right away she'll become a cow, but Mahiro, already half asleep, just makes a mooing sound in response.

Mihari remarks that complete rehabilitation seems to have a ways to go.

Characters

 * Mahiro Oyama
 * Mihari Oyama
 * Kaede Hozuki
 * Momiji Hozuki (mentioned)