Bullying One's Sister

There is one so-called "problem student" in the 8th-grade class where I've been conducting participant observation. He once told me that he didn't have any siblings; however, it turned out that he has a sister in one of the 6th-grade classes that I've been observing. When I talked with his sister, she told me how she feared his brother because he bullies her. The reason he bullies his sister is probably that he is frustrated with pretty much everything in the world, including himself. So, he bullies her sister to displace his frustration.

But why is he frustrated? According to him, his father has been very harsh toward him, which makes him angry and full of hatred. That is, the father bullies his son who in turn bullies his sister. But his sister is not bullying anyone partly because she has no younger sibling and partly because she is weak. Perhaps she has become a weak person because of the bullying by her brother.

I just hope that she will not become a mother who bullies her children. Since I am going to visit her class tomorrow, I will cheer her up.