Yeah, those kinds of nicknames are generally reserved for older siblings or the older generation towards someone younger. And a nickname like Yanyan is very affectionate, so no one would even call her that out of disrespect (unless someone was disrespectfully trying to flirt). There's exceptions, mostly in modern usage, but that's the rule for the most part. It's a bit nebulous honestly! Most of the time, I just go by what the canon has them call each other.