>>280061367 (OP)Because she's an adult and muh japanese beehive mindset.
Arata gonna step in and Banri will get assmad at his OTOUTO for being a kuso namaiki gaki meddling into adult issues he would never understand, when the reason Arata is definitely going to meddle is because he, as the dependable brother, will obviously grow a hateboner against anyone who dare to do something bad to anyone of his sisters against their will.
This shit is obviously going to create a misunderstanding. And to make things worse, Ouka behave like a fucking callous brat when she openly fantasized about Banri's relationship with her boyfriend instead of reading the room.
As fsr as i saw in 5ch, Ouka became too disliked there for being too self centeted and whiny. And this manga chapter sure made them hate her more.