This root had a variety of meanings over Tolkien’s life. It first appeared as ᴹ√NETH “young” in The Etymologies of the 1930s with various Quenya and Noldorin derivatives of similar meaning, the most notable being the name ᴹQ. Nessa (Ety/NETH). In rough notes probably from around 1959, Tolkien redefined √NETH or √NES to mean “feminity apart from sex”, contrasted with √WEG or √WEƷ for masculinity, but he abandoned this line of reasoning deciding that √WĒ simply meant “person” independent of gender (PE17/189-190).
In notes on Eldarin Hands, Fingers and Numerals from the late 1960s, Tolkien again redefined √NETH, this time as the basis for Elvish “sister” words, either specifically meaning “sister” (VT47/12, 26, 34), meaning “(young) woman” (VT47/15, 32, 39) or perhaps “fresh, lively, merry” (VT47/32), the last of these probably a callback to its 1930s meaning “young”. In notes from 1959-60, however, the name Q. Nessa was disconnected from the root √NETH, derived instead from ✶Neresā meaning “she that has manlike valour or strength” (WJ/416).
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is best for √NETH to retain its essential 1930s meaning “young, fresh, lively”, but with an association to young women as described in Tolkien’s notes from the 1960s, and thus by extension “sister” as named from the perspective of the parents.
An unglossed root in the Outline of Phonology from the early 1950s illustrating certain phonetic combinations (PE19/98), and therefore possibly not a “real” root.