@shanoc5902 I won't deny there is a possibility that the brother did something horrible, assuming the artist capture the image as the author wants. However, I still have a little doubt. If, a big if but...if the brother is actually a good person. It could be that in chapter 9 page 11:
1. he visited her school.
She is a high school student. So she is a kid around 14-16 yo, living in a dorm/alone. I think it is normal that the parents or older siblings check up on her. See if she is living a hard life, like insufficient money, low-quality food or bad facilities and all that. Or simply looking if she is adjusting well to living alone. Not to mention the reason she wants to live outside her parent's house is to not be a hindrance to her older brother. Imagine the older brother knowing that. Wouldn't he feel sorry or worried?
Also, even if she is cheerful on the outside. At home, she seems quiet. Maybe the brother wants to see if the quiet sister is being bullied. To him, it could be the real reason she chooses that school.
2. he came to her house with a key she didn't give him
Looking at the hair length and the face when she is in high school, page 11, and university, page 12. It seems that it is still at the time when she lives alone and still in high school.
We don't know her house neighborhood condition, we don't know if she regularly keeps contact with her parents. With this assumption, it might be that the brother came, either by his own initiative or by the parent's order. He can talk to the security/whoever holds the spare keys that her highschool sister lives alone and he wants to check up on her. As proof, he can show his ID, family registration form and family photo. I think this is how he got in with a key.
3. broken glass and "Nobody was watching"
There is a possibility she reacts badly to her brother intruding her into her house. They have a fight. Maybe the brother is at fault here, venting his emotion and getting angry on how her actions on wanting to live alone and maybe beginning to stopping all contact to her family is making the family worried or seen badly in public or to him her actions of avoiding him like a plague just don't make sense at all. Afterall on page 12, she said she purposefully made her self hidden and hard to track. So yes, "nobody watched" the usually kind brother loses his cool and explodes. It scared her even more.
It could be that the brother is a really twisted sick guy. However, until that is confirmed, I am more inclined on she was not able to see her brother real nature, jump to conclusions and too blinded by prejudice or constant low self-esteem caused by toxic environments growing up.