The root √YEL was one of several competing Elvish roots for “daughter”. In The Etymologies of the 1930s ᴹ√YEL “daughter” was first used as the basis for ᴹQ. yelde/N. iell “daughter”, but it was deleted (Ety/YEL). N. iell was given a new derivation from ᴹ√SEL-D “child”, by analogy with N. ionn “son” (Ety/SEL-D), while a new Quenya word for “daughter” was introduced: ᴹQ. yende from a feminine variant ᴹ√yēn of ᴹ√YO(N) (Ety/YŌ). Note that ᴹ√SEL-D itself was initially glossed “daughter”, but was changed to “child” and given derivatives for all genders in Quenya: ᴹQ. selda [n.], ᴹQ. selde [f.], and ᴹQ. seldo [m.].

The picture in later writings is also rather muddled. 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). In several places Tolkien gave Q. Tindómerel “Daughter of Twilight” as the Quenya equivalent of S. Tinúviel, with the final element being derived from primitive ✶-sel(dĕ) > -rel (Ety/SEL-D; PE19/33, 73; VT47/37).

In this period, however, the more common suffix for “daughter” was Q. -iel as in Q. Elerondiel (S. Elrenniel) “*Daughter of Elrond” as applied to Arwen (PE17/56) and Q. Uinéniel “Daughter of Uinen” (UT/182). Furthermore, in a list of masculine and feminine suffixes written around 1959, Tolkien gave (primitive?) yē, yel and (Quenya?) yelde for “daughter”, though in that note the feminine patronymic suffixes were revised from {-yel, iel, -yelde >>} -well-, -uell-, -wend-, -wel, and yen was given as another variant (PE17/190). In other notes from the late 1950s associated with “Changes affecting Silmarillion nomenclature”, Tolkien had feminine patronymic suffixes -en, -ien, but said that Quenya used -ielde, -iel (PE17/170).

Neo-Eldarin: All of the above indicates considerable vacillation between √SEL, √YEL, and √YEN for “daughter” words and suffixes in the 1930s to 1960s: of the three Tolkien seem to favor sel- for “daughter” words but -iel for “daughter” suffixes. For purposes of Neo-Eldarin, I would assume ᴹ√SEL(D) originally meant “child”, with √YEL an early variant meaning “daughter” under the influence of √YON “son”, especially used as a suffix. However, due to reverse influence Q. seldë and S. sell were early on used to mean “daughter”, with female child = “girl” words becoming Q. nettë and S. neth.