Tolkien gave this root in The Etymologies of the 1930s as ᴹ√THEL and extended form ᴹ√THELES with the gloss “sister” and derivatives like ᴹQ. seler and N. thêl of the same meaning, both derived from the extended root as made clear by the Noldorin plural thelei < ON. thelehi (Ety/THEL). Hints of the roots continued use appear in the 1959 term Q. meletheldi “love-sisters” for close female friends (NM/20). In notes from the late 1960s, Tolkien gave Q. nésa and S. nethel as the words for “sister”, both from the root √NETH. Nevertheless, I think it is worth retaining ᴹ√THEL(ES) to represent more abstract notions of “sisterhood” for the purposes of Neo-Eldarin, for “metaphorical sister”s as opposed to Q. nésa/S. nethel for sisters by blood.
Middle Primitive Elvish
sel(d)
root. child, child; *daughter
thel(es)
root. sister
yel
root. daughter
galan
root. bright
kyab
root. taste
ni
pronoun. I
wen-
noun. girl
yen
root. daughter
yend
noun. daughter
A root in The Etymologies of the 1930s, initially glossed “daughter” but later “child” with derivatives ᴹQ. selde, ᴹQ. seldo, ᴹQ. selda = female, male and neuter “child” (Ety/SEL-D). In Notes on Names (NN) from 1957 Tolkien gave sel-de “daughter” (PE17/170), while S. sel(l) = “daughter” appeared in both the King’s Letter from the late 1940s (SD/129) as well as the Túrin Wrapper from the 1950s (VT50/5). The diminutive form for “daughter” appeared as Q. selyë in notes from the late 1960s (VT47/10).
Neo-Eldarin: For purposes of Neo-Eldarin, I prefer √YEL for “daughter” as a variant of ᴹ√SEL(D) under the influence of √YON “son”, mostly so I can still use the 1930s “child” words for other genders, at least in the Quenya branch. I would still use Q. seldë and S. sell for “daughter”, however, with a bit of semantic drift, with “girl” words becoming Q. nettë and S. neth.