I think it's funny that Momo refers to her brother as eldest son instead of his given name. Maybe that's more common in Japan, but it's done here mostly for convenience, since the franchise hasn't given Momo's relatives any official names yet.
Hand-me-downsI received some hand-me-downs from the President and Yuzu-chan...Otou-chan?I received some hand-me-downs from the President and Yuzu-chan...The ones from the President are fine,
but it seems like Yuzu-chan's won't fit huhSince it'd be a waste, the eldest son should wear itEh!?Th-The clothes that Yuzu-neechan wore...What if that turns into a strange hobby?!!
I'll take it instead!!
Spouses often refer to each other the same way their kids do.