Well the whole damn family didn't have any interest in her other than that second brother, and even that second brother interest is just like the interest you have when you saw an ant in a street
It may be because he is just used to her smiling like that. I am not sure how much older he is than her. But if as a child himself that was all he was used too it would take her dropping him cold turkey for him to notice.
So like she is 15(She looks older cuz the art style but she is just 15) lets say he is at best what 2, maybe even 4 years older? His dad is not interested in her, so he has little interest either. And so he grew up not understanding how fucked up his family's dynamic with the daughter is.
How come he‘s only noticing now that her smiles were fake? Its so freaking obvious in children