Yao: The last one. Used to describe the smallest one. Because "Yao" means "small", "tender" and "young", "Yao sister" means "young girl" or "beautiful little girl", which generally means a kind of favor.
Therefore, if you call your girlfriend your sister, in many cases, it should be a pity and love for your girlfriend. It belongs to the unique love, pity, cherish, love, concern and emotional elements of love between lovers.