Dude, lol. She told him during waterfall scene.. which is after father told them that they're not related. So no incest. Arata is the one who views girls as sisters not other way around.. lmao. So, you're literally saying that Banri is pushing that "incest" because she's the one who confessed to him (so are others girls pushing incest since Arata did nothing wrong with them). Kotono - she kissed him. Ouka - she kissed him. Minami confessed to him. Seiha was straddling him etc. Arata did literally nothing with any of them. They were coming at him..
Yes.. Banri's disloyal to Arata. She literally doesn't deserve him. Arata deserves girl equal to him. Arata will always continue looking at her(Banri) as sister as he said in recent chapter. That's why Ouka will be his wife, I wouldn't mind Minami, though.