(in gwanûn) (WJ:367). Also gwanur (in gwanur) (LotR Appendix A). Note: a homophone of gwanur means ”brother; kinsman or kinswoman”
Sindarin
gwanûn
noun. a pair of twins
gwanon
noun. twin
gwanunig
noun. a twin (one of a pair of twins)
gwanunig
noun. twin
gwanûn
pair of twins
gwanur
pair of twins
(in gwanur), also gwanûn (in gwanûn) (WJ:367)
gwanunig
twin
gwanunig (i **wanunig**), a singular formed from:
gwanunig
twin
(i ’wanunig), a singular formed from:
gwenyn
noun. twins
gwenyn
pair of twins
(PM:353, 365)
The Sindarin word for a “twin”, with a plural form gwenyn using the ordinary pluralization rules of Sindarin (PE23/140; PM/365). It is likely a combination of gwa- “together” and primitive ✱nonā “born” based on the root √NŌ; compare Q. onóna “twin-born” of similar derivation (WJ/367). It also had a specialized dual form gwanûr for a pair of twins (LotR/1054), derived instead from ✶wo-nōrē “together-birth”, functionally a singular noun in archaic speech but in modern speech treated as a variant plural applicable only to a pair of twins (PE23/140). Gwanûr is thus probably used more often than gwenyn.
Conceptual Development: The Gnomish Lexicon of the 1910s had G. gadwi “twins”, a dual form of G. gad(a) “link” (GL/37), along with a longer variant G. hethgadwi or heth-gedwi that had heth “sibling” as a prefix (GL/49). The Quendi and Eldar (Q&E) essay from 1959-60 had another dual form gwanūn, probably based on ✱wo-nōnā, from which a singular gwanunig was derived.
Neo-Sindarin: For purposes of Neo-Sindarin, I would use gwanon as the singular, gwanûr as the dual, and gwenyn as the ordinary plural, since these forms appear in either The Lord of the Rings (LotR/1054) or in notes from 1969 (PE23/140; PM/365).