Tolkien used a similar set of Elvish roots for “youth” and “freshness” for many years. The earliest of these was primitive guı̯u̯ or gu̯iu̯ [ᴱ√GWIWI] in the Gnomish Lexicon of the 1910s with derivatives like G. gui “just, just now, only just, already”, G. guin “recent, fresh”, and G. gwioth “youth” (GL/42). This root reappeared as ᴹ√WIR “new, fresh, young” in The Etymologies of the 1930s with variants ᴹ√WĪ and ᴹ√WIN and derivatives ᴹQ. vírie “youth” and ᴹQ. virya “fresh” (EtyAC/WIR). The ᴹ√WIN variant had derivatives ᴹQ. vinya/N. gwîn “young”. Tolkien considered, but rejected, deriving these from strengthened ᴹ√GWIN instead, producing (also rejected) ᴹQ. winya/N. bîn (EtyAC/GWIN).
Q. vinya appeared in quite a few later names with the gloss “young” or “new”, but the Sindarin form became S. gwain as in S. Narwain “January, ✱(lit.) New Fire” (LotR/1110) and S. Iarwain “Old-young” (LotR/1114; RC/128). In Definitive Linguistic Notes (DLN) from 1959, both were given as derivatives of √WIN “young” along with Q. víne/S. gwîn “youth”, though the Sindarin word for “young” was given as (archaic?) gwein (PE17/191). Also related are various words for “baby” from 1968 notes such Q. †wine/S. gwinig “little-one, baby” (VT48/6). In these notes primitive wini was glossed “little” but this was deleted (VT47/26), making it likely that the earlier senses “young, new” were restored for √WIN.
As for the 1930s root ᴹ√WIR, it might have survived as an element in the month names Q. Víressë/S. Gwirith “April” (LotR/1110), perhaps meaning “✱freshness”.
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.