A word for “sister” coined by Tolkien in notes on Eldarin Hands, Fingers and Numerals from the late 1960s, based on the root √NETH of similar meaning and replacing the archaic form of the word †nîth (VT47/12, 14). The diminutive/affectionate form nethig “[little] sister” was used as a play name for the fourth finger (VT48/6); Tolkien considered an alternate diminutive netheg (VT47/14, 32) and also considered giving this diminutive an alternate meaning “little girl” (VT47/15, 33); see S. neth for discussion.
Conceptual Development: The Etymologies of the 1930s had N. thêl “sister” from the root ᴹ√THEL or THELES (Ety/THEL), and the Gnomish Lexicon of the 1910s had G. hethir “sister” from the early root ᴱ√HESE [HEÞE] (GL/48; QL/40). See those entries for discussion.
A word appearing in several sets of notes from the late 1960s having to do with Hands, Fingers and Numerals. It was related to the finger name of the fourth finger (representing the sister). Tolkien gave it a variety of glosses: “girl” (VT47/33), “little girl” (VT47/33), or as an archaic diminutive for “sister” (VT47/14; VT48/6), where it was replaced by nethig in modern speech. He also gave it several different derivations, from √NET “trim, pretty, dainty” (VT47/33) or the root √NETH which itself was given a variety of meanings: “sister” (VT47/12, 26, 34), “(young) woman” (VT47/15, 32, 39) or “fresh, lively, merry” (VT47/32).
Neo-Sindarin: For purposes of Neo-Sindarin, I think the meaning “(little) girl” is the most useful. For “sister” I would use †nîth or nethel, with nethig as the diminutive form.