Since you’re an older sister too, one day Taiju asked you if you could teach him how to be like you, as an older bother. But you were helped by the fact that Yuzuha, the middle sister, was your best friend so you often go to their house and after spending some time with Yuzuha, you could teach him something

“I don’t understand, why I have to do like this? They will get weaker”

He asks him one day, while you two were sitting next to each others in front of a textbook