It's in narration text on page 4 that he thinks she’s a younger sister.Let's see how common this trope is.
Note that when seeing the mom, he didn't actually say he thought she was the little sister.
Also what's up with the "leaving stuff to you on the class trip"? I went back and re-read and that doesn't make sense... the only thing was he left her when she was cooking, and he had no choice because the teacher called him.