Gnomish
heth Archaic
noun. brother or sister
heth
adjective. white, pallid, wan
hethgadwi
noun. twins
hethgedwi
noun. twins
gadwi
noun. twins
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
An archaic term (or possible a root) in the Gnomish Lexicon of the 1910s for “brother or sister” [sibling] (GL/48). It is clearly related to the early root ᴱ√HESE in the Qenya Lexicon which has various “brother” or “sister” derivatives (QL/40). Its dual form hethwi appeared in the Gnomish Grammar with the gloss “a brother and sister” (GG/10).