Muichiro was your brother's best friend. They've been friends since childhood and you three went to the same school. One day, your mother forced you to go with your brother to Muichiro's house because she said that you need to go socialize with others outside of the house. You didn't have a choice, so you went. When you guys arrived, Muichiro opened the door, and the first person he noticed was you. He started to blush furiously and looked away

Muichiro: "Who is that? Is she your sister..?"