“High School Boys and Exchanges” is the 94th chapter of the manga.
Motoharu and his sister are walking home and she asks him what he wants for dinner that night. He says sukiyaki udon. Out of the blue, she tells him to start doing his homework on his own instead of depending on her. Puzzled, he tells her he does do it all on his own. She says it needs to look like they quarrel once in a while, so that others think they’re on good terms.
This perplexes Motoharu, and he asks her what sort of lies she had been telling her friends about him, reiterating that he never asked her for help with homework. She brushes it off, annoying him. They then continue walking. Motoharu, noting that they’re discussing siblings, says that he’s always wondered what it would be like to have an older sister. His sister is agape and she asks him what he meant.
Starting as if in realization, he exclaims he already has an older sister. His sister asks him if he meant to say that she was never really an older sister to him, but just a vaguely familiar individual in his house. When he agrees, she throws a punch at him in rage, but he ducks and apologizes.
As they continue walking, Motoharu’s sister tells him she can do something about it if he really wanted an older sister that much. She places a phone call to someone and starts informing them about the situation, much to his reluctance. After the call, she tells him she’s arranged for an older sister to come over to their house for a day, while she goes to the other house as the older sister there.
This doesn’t sit well with Motoharu, but he keeps quiet to avoid upsetting her. Later, at his house, Motoharu finds that he has Hidenori’s older brother for company. He asks him if he’s supposed to be the ‘sister’, and he says yes, much to Motoharu’s confusion. Yūsuke says it’s a long story and explains that Yoshitake had wanted an older brother, so he and Hidenori first exchanged siblings, after which Yūsuke got a call from Motoharu’s sister. At this point, he is cut off by Motoharu, who says it’s too much trouble to keep up.
In order of appearance: