Tadakuni's younger sister/Relationships

Tadakuni
They aren’t exactly on good terms. The two are claimed to talk to each other only around once a month. Tadakuni, well aware of her strength and violent nature, tries not to get on her bad side. However, she is extremely irritated when he and his friends mess around with her underwear, first attacking him and his friends and later complaining to Yoshitake’s older sister. She also seems to lack confidence in him, going to other boys in his school for help when she was in trouble, rather than talking to him.

On the other hand, she often eavesdrops on him and his friends and gets carried away by his stories.

Yoshitake and Hidenori
Tadakuni’s younger sister primarily views the pair as a nuisance, as they and her brother typically play around with her underwear. However, she also seems interested in their activities, eavesdropping on them when they talk to her brother. Like a few others, the two refer to her as simply ‘imōto’ (younger sister).

She is also seen interacting with Hidenori when asking him to help get her bag down from a tree and when they both repeatedly slip and fall on ice while going to school.

The two boys eavesdrop on her in turn when she visits North High and later follow her to make sure she’s safe, although Yoshitake (by his own admission) was looking to ‘play the hero’ more than anything else.

Karasawa
Tadakuni’s younger sister seems to harbor a liking for Karasawa. She declared that out of her brother’s friends, she’d rather date him than anyone else. Also, when Tadakuni claims to have seen something hilarious under his cap, she chases him down to find out what it is, although she ended up dismayed by her discovery.

Later, when she gets into a fight with a boy at her school, she visits North High seeking his help and appears very disappointed when he says she should sort it out herself. The latter incident could just have been a ruse for her to visit Karasawa, considering her above-average strength and ability to manhandle Karasawa himself.

Yoshitake’s older sister
Despite their age gap, the two are linked by a mutual disdain for the main trio’s tomfoolery (and a tendency for violence). Yoshitake’s older sister is made aware of the boys’ misadventures with Mei’s underwear. They seek out Yūsuke together and ask him to help curb the menace, later ganging up on him when they find out he’s involved as well.

Tadakuni’s younger sister also shows alarm when Yoshitake’s sister considers dating Yūsuke.

Other
In the last chapter, the sisters of Tadakuni, Yoshitake and Motoharu were shown visiting the school festival together, implying Tadakuni's younger sister's acquaintance with Mino.