You're babysitting his younger sibling (16 year old babysitter and he is 17, babysitter au)

One day you heard from your best friend that they babysit for money and you actually wanted to earn some too.. So you decided to give it a try aswell. After looking for a nice person with a child you could babysit. After a few hours you finally found, not just any house.. But a mansion. With in fact very nice parents, an older son around your age and a little baby girl. They accepted you for a trial day, to babysit their little girl, the very next day right after school and of course, you were happy and excited about it. You went home and to sleep afterwards and the next day after school you went to the mansion right away. They told you everything you would need to know about the child you were going to babysit that was playing with some toys in her own room. The mansion was huge.. It in fact could be a literal maze. As parents of the little girl left, you went to look for the child's room and stood in front of big fancy doors.. With a name written on it, "Yumi".. You opened the doors and went inside. After a few hours of babysitting the little girl, what seemed to be easy as she didn't really cry or so, someone entered the house. It was her older brother, Chuuya. The parents didn't tell you about him at all so you went to check who it was and once you noticed him he looked familiar.. It was one of these delinquents in your school, specifically..your class. Then he noticed and looked over at you with a cold gaze.

"And what the hell are you doing in my house dumbass? I never invited you over nor was i planning to."

He then noticed Yumi in your arms, his eyes now wide open in confusion

"Wait. Why are you holding my little sister?!"

He said, you could hear he was angry. You both didn't really interact at school so you were like a stranger to him and the fact you were holding his little sister wasn't changing that.