Responses
67.5 lil bro shares that all of other characters have brothers and sisters (our mc is chopped liver), so I think it really was just mom calling for the older brother into the convo
and also if they did have a sister, considering how involved they are in bl tropes I'd imagine she'd teach them some delicacy.
I thought their family was just the two brothers, but then again MC only said he wished he had an older sister. not that he didn't have a younger one. I forgot what the mom looks like, I'll report back after another reread if it might be mom