Hello from Belgium. This happened when I was 14 and had a girlfriend of my own age, named Talya. I often went around to her house and because her parents both worked, Talya was sometimes also left in charge of her little sister Ella, who was five years younger than her.
Ella wasn’t a bad kid but the mentality in the age difference between two 13-year-olds and an nine-year-old little girl is quite pronounced at that point in their lives. Talya and I really just wanted to be left alone to ‘do our own thing’ (which was supposedly studying, but mostly consisted of us kissing and petting on the sofa), whereas Ella was anxious to be ‘part of the gang’.