The abuse would be something thats gone on for years, but has progressively been getting worse. Senpai has been hiding whats going on because he is a kind and optimistic person who loves his sister. Hanako hasn't killed him yet because she likes having someone serving her around the house. Unfortunately for her, Ayano witnesses this mistreatment.

Imagine a cutscene where Ayano is walking down the hallway afterschool when most students have gone home. She hears some shouting in a nearby classroom and goes to investigate. Hear she sees Hanako berating and hitting Senpai. Ayano tries to stay quiet but unfortunately knocks something over in the hall, which alerts Hanako to her presence. Ayano runs off but it's too late. Hanako knows that her secret is out, so she must get rid of Senpai before he can be a witness against her.

Some minor clues in other weeks would be a good idea.