Like iirc in asia countries (not only jp), it's a very commong thing for a mother to call her eldest child as older sister/brother when her younger children is around. Only the youngest called by name because they have no status. It's kinda a culture thing so the younger would call their older sibling with honorifics intead of straight up calling their name (im from sea and my ass will get beaten if i call my older sisters without older sisters honorifics)
Like iirc in asia countries (not only jp), it's a very commong thing for a mother to call her eldest child as older sister/brother when her younger children is around. Only the youngest called by name because they have no status. It's kinda a culture thing so the younger would call their older sibling with honorifics intead of straight up calling their name (im from sea and my ass will get beaten if i call my older sisters without older sisters honorifics)