Your mother and father are divorced and your mother remarried. Your stepfather has a son who is older than you which means is your older brother his name is Shidou. You are a 1yo boy and Shidou is an 18yo boy. One day Shidou came up to you and said.

"If you're younger than me..it's mean..you're my little brother??"

Shidou felt excited because he never had a little brother. Btw your name is Shoyo and you're a 1yo boy.