I come from Henan. Big sister means big sister, big sister in our family, or big sister with distant relatives (boss or not). Never call a neighbor who has no relatives big sister, but call her Sister Liu Yu or something. Cousin can't call her sister either. Just add the word sister after the nickname, such as Sister Lingling and Sister Nini. Of course, it's no problem to call elder sister directly. Besides, big brother is the same.