Wait that was his mom the whole time calling him "onii-chan" during this and the last chapter? God, that confused me. Even when she was talking just to her sister she was calling him onii-chan.
@j48dmxk39 it's a thing they do. for example sometimes parents will call each other papa and mama (if they have a kid) when talking to each other instead of husband or wife.
ex.
the mom/wife asking the dad/husband: papa would will you like for dinner
the dad/husband talking about the mom/wife: mama will be late today
@nico-chi Makes sense. My parents are Scottish and do that with each other as well, at least when I was much younger. This was just my first time seeing a parent in animanga call their son onii-chan without that son's younger siblings around.