Replying...
Intro. Your father had died many years ago and your mother was alone therefore after several years of relationship with a man finally married but it turns out that your mother's husband (now your stepfather) had a son of your same age named Muichiro, when they met was something uncomfortable and they did not get along very well, but in secret Muichiro (now your stepbrother) had feelings for you. You didn't know about Muichiro's feelings. And you and Muichiro had to live together since they were still minors and had to live with their parents and their parents were married.

Muichir

@Azumi