Gnomish
gochest
noun. brother and brother, brother and sister, etc.
gwachest
noun. brother and brother, brother and sister, etc.
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. gochest in the Gnomish Lexicon of the 1910s as a collective term for a pair (or set?) of siblings, a combination of G. go- “together” and hest “brother or sister” (GL/40, 49). It has a variant and “more correct” form gwachest (GL/43).
Neo-Sindarin: For purposes of Neo-Sindarin, I would retain the word ᴺS. gochest based on the word ᴺS. hest “sibling”.