Jinnai Siblings

Jinnai Siblings is the family ship between Enta Jinnai and Otone Jinnai from the Sarazanmai fandom.

Canon
In Episode 1, Otone tells Enta to not waste money. They argue for a bit, before the box starts inexplicably floating, leaving Otone dumbfounded and Enta screaming for it to come back. In Episode 2, Otone sends a photo of her and her boyfriend to Enta. The latter wonders why she sends that, as he's alone and worried. It foreshadows Enta's crush on Kazuki.

In Episode 3, Enta starts getting nervous about "kisses", as he did kiss Kazuki while he was asleep. Otone says that she is going on a date with her boyfriend, and gets excited when the Lucky Item of the Day is a kiss. Enta wonders when did he get that crush on Kazuki. He daydreams and accidently gives the miçanga to his sister. She thinks that it's for her, and goes on looking for her boyfriend. Enta realizes that, and start chasing after her, before getting distracted by other middle schoolers playing (and stealing) the Golden Duo's spot. He eventually gets the miçanga back, only for a mouse to steal it.

In Episode 9, Kazuki tells the adults that a policeman shot Enta, but everyone believes that it's Chikai. Otone asks Kazuki to stop. She tells him that Chikai will be arrested and everything will be okay. She confesses that Enta was happy because of Kazuki, as their parents and herself weren't always there for him. She says that she is glad that Enta made a friend like Kazuki, which makes the latter realizes how much Enta is important for him. As he runs towards Enta's room, she finally cries.

They don't have much interactions in the series, but one can notice that they are pretty similar. Both are based on Lulu Yurigasaki (a character from Yurikuma Arashi, another Ikuhara anime) :Otone has her appearance and Enta has her character arc. Both are lustful for boys, as Otone shows it in Episode 3. Both are sporty :Enta being on the soccer team and Otone being a P.E teacher. And in the manga, both have the same way of opening a door.

Fanon
In the fandom, Otone is headcannoned as bisexual, with Enta being supportive. Even though they're never the center of fanfics, their dynamic stays pretty faithful to the canon, with Otone just being a bit more teasing. She's supportive as well, and also takes care of Enta's partner(s) emotionally. Enta stays pretty much the same, but more interested in his sister's life. In other words, it's pretty heartwarming.

Fandom
Jinnai Enta & Jinnai Otone on AO3

Trivia

 * Otone was originally going to be a main character alongside her brother.