Gnomish
hethrin
adjective. of the same family (in a close sense), consanguine
renn hethrin
first cousin
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 in the Gnomish Lexicon of the 1910s glossed “of the same family (in a close sense), consanguine”, an adjectival form of archaic G. †heth “brother or sister” (GL/48). Tolkien specified that it described those “usually having both parents same, but also occasionally = having two grandparents alike”.