Middle Primitive Elvish
neth
root. young
nethrā
adjective. young
thel(es)
root. sister
indise
feminine name. bride
ndīse
noun. bride
neñwi
noun. nose
wen-
noun. girl
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.
Tolkien gave this root in The Etymologies of the 1930s as ᴹ√THEL and extended form ᴹ√THELES with the gloss “sister” and derivatives like ᴹQ. seler and N. thêl of the same meaning, both derived from the extended root as made clear by the Noldorin plural thelei < ON. thelehi (Ety/THEL). Hints of the roots continued use appear in the 1959 term Q. meletheldi “love-sisters” for close female friends (NM/20). In notes from the late 1960s, Tolkien gave Q. nésa and S. nethel as the words for “sister”, both from the root √NETH. Nevertheless, I think it is worth retaining ᴹ√THEL(ES) to represent more abstract notions of “sisterhood” for the purposes of Neo-Eldarin, for “metaphorical sister”s as opposed to Q. nésa/S. nethel for sisters by blood.