The popularity of this title is mainly because it is more in line with the current public taste!
First of all, the appellations of "elder brother" and "elder sister" mostly appear among children, which are relatively intimate and cute ~ ~ ~ and the era of being called handsome and beautiful two years ago has passed.
Adding a word "Xiao" in front of it makes the addressed person look kind, lovely and simple, like "Xiao Cute", "Xiao Cubi", "Xiao Qing Xin" and "Xiao", but it doesn't feel like making friends on purpose. If you pass by, you will be called "Sister", and it is inevitable that a lady who doesn't know the truth will think that you have mistaken me for someone else, or that you have slipped out of a mental hospital.
Five thousand years of Chinese culture, each era has its popular name, so the popularity of "little brother" and "little sister" is the inevitable trend of historical development! Once popular titles such as "handsome boy" and "beautiful girl" will gradually be found only in TV dramas or Baidu websites, just like ancient titles such as "girl" and "miss".
For boys, calling girls "little sisters" is bound to add a lot to the skills of picking up girls. There is neither a sense of distance to directly call "classmate" nor a sense of disobedience to call "little cute", which is perfect, so it is completely correct to call Miss Sister whether it is to close the distance or ask someone for help ~ ~ ~