Okay so it's the brothers fault. it kind of doesn't make sense for her to want to fight the female protagonist she should be trying to talk to a fight the brother because he's the root of all of this.
God, so good. With any other manga, the backstory would just be some cliche sob story. Here, it actually connects and gives more depth and understanding on why they're acting the way they are.
sanae didn't really bully the brother at all, but everything is all warped in natsuo's mind in her quest to defend her brother - really emotional backstory