@skyron
Since she actually is 9 years old, why not just straight up call her something like "9YO Little Sister"? Calling her "jiumei" is extremely confusing(imo) since it
usually means "Ninth Sister".
I'm actually curious where you found those 2 bubbles stating that since I looked over the raws for chapter 3.
Edit: Nevermind, I found the 2 bubbles you were talking about(in Chapter 1) and I still stand by either calling her "Ninth Sister" or "9YO Sister". The first mention is 九小姐(Ninth Young Miss/Ninth Miss) and it doesn't seem to be an issue. The second is the 9-year old mention on the next panel.
I get that you believe that there is a correlation between the age and status, but if you take into account the fact that it's a servant giving the news to the Patriarch, I don't think a servant would refer to Nie Yurou by her age unless the 2 had a bad relationship. Maybe I'm just not watching enough Chinese dramas, beats me.
Also a note: it's not uncommon that clans in settings like these have multiple siblings. They could all come from different mothers, yet still be nicknamed after the order they were born. First born = First Brother/Sister, second born = second brother/sister, etc etc.