A noun in The Etymologies of the 1930s glossed “sister, associate”, the female equivalent of N. gwador “(sworn) brother, associate”, a combination of prefix N. gwa- “together” with the root ᴹ√THEL “sister” (Ety/THEL). It is used for metaphorical “sisters” by oath or association, and not by blood.
Noldorin
thêl
noun. sister
muinthel
noun. sister
muinthel
noun. sister
thêl
noun. sister
gwathel
noun. [sworn] sister, associate [f.]
gwathel
noun. sister, associate
gwador
noun. brother (especially used of those not brothers by blood, but sworn brothers or associates)
gwîn
adjective. young
muindor
noun. brother
muindor
noun. brother
neth
adjective. young
neth
adjective. young
irregular, compare: tathor @@@
tôr
noun. brother
The word muindor is more usual
tôr
noun. brother
An (archaic) word for “brother” in The Etymologies of the 1930s derived from the root ᴹ√TOR of the same meaning, with an irregular plural terein (Ety/TOR). In ordinary speech, it was replaced by muindor, with an initial element muin “dear”.
Neo-Sindarin: In notes from the late 1960s, Tolkien introduced a new word hanar for “brother” (VT47/14). However, I think †tôr and related words might be retained to mean a “metaphorical brother”, a close male associate who may or may not be related by blood, as with such words as gwador “(sworn) brother, associate”. In this paradigm, I would assume muindor still refers to a brother by blood, with an added connotation of strong affection.
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.