A word for “sister” in The Etymologies of the 1930s derived from the root ᴹ√THEL or THELES of the same meaning, with an irregular plural thelei (Ety/THEL). It had a more elaborate form muinthel, the equivalent of muindor “(dear) brother”, with an initial element muin “dear”.
Neo-Quenya: In notes from the late 1960s, Tolkien introduced a new word nethel for “sister” from the root √NETH (VT47/14). However, I think thêl and related words might be retained to mean a “metaphorical sister”, a close female associate who may or may not be related by blood, as with such words as gwathel “[sworn] sister, associate”. In this paradigm, I would assume muinthel still refers to a sister by blood, with an added connotation of strong affection. I think it’s best to assume the irregular Noldorin plural pattern was reformed to the normal Sindarin plural thîl.
An adjective in The Etymologies of the 1930s given as N. hethw “foggy, obscure, vague” derived from primitive ᴹ✶khithwa under the root ᴹ√KHITH “mist, fog” (Ety/KHIS; EtyAC/KHIS).
Conceptual Development: G. huir “foggy” from the Gnomish Lexicon of the 1910s is a likely precursor (GL/49), perhaps derived from a (hypothetical) early root ✱ᴱ√HUYU.
Neo-Sindarin: If adapted to Neo-Sindarin, this word would become ᴺS. hethu, as suggested in HSD (HSD).