Gnomish
hest
noun. brother or sister, *sibling; †consanguinity
hothron
noun. captain
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.
A word appearing as G. hest in the Gnomish Lexicon of the 1910s for “brother or sister” [sibling], but “properly consanguinity, the relationship of brother and sister, or brother and brother, etc.” (GL/48); this original meaning seems to be archaic, transferred to G. hestri or G. gochestri. This word is derived from the early root ᴱ√heth (GL/48).
Neo-Sindarin: For purposes of Neo-Sindarin, I would retain the word ᴺS. hest as a derivative of the Neo-Root ᴺ√KHETH to serve as a gender-neutral term for “sibling”.