So with Avatar SE being released in ten days (F*** YES), we've already seen a few bits and pieces of the extra 9 minutes of footage. One in particular, is Jake visiting Grace's old school, which lead practically all of us to theorize on Sylwanin's existence. The most important questions would be her age relative to Neytiri's, as well as if she's alive or deceased.
However, in my 14th viewing of the movie, I noticed a few bits in the background of scenes that lends credability to her existence, being Neytiri's younger sister and likely deceased.
Note this picture. It's a shot of Grace's Na'vi students, including Neytiri. Beside Neytiri is a younger Na'vi girl who looks suspiciously similar to Neytiri.
Now look at this picture. It's Jake and Neytiri on the way to the Tree of Voices.
Compare the loincloths of the unknown Na'vi girl in the 1st picture to Neytiri's in the 2nd picture. Look similar?
Given the position of everyone in the shot with Grace's school, it's likely Sylwanin is the girl posing besides Neytiri. And Neytiri's loincloth (already seeming to be too small for her, which is corroborating evidence that it wasn't tailored for Neytiri exclusively) at the Tree of Voices may be her wearing it in rememberance of her dead sister.
Granted, it may not be exactly the same, but I'm guessing that the loincloth was slightly altered so it would actually fit on Neytiri's waist.