A noun for “sister” in The Etymologies of the 1930s derived from the root ᴹ√THEL or THELES of the same meaning, with an irregular plural selli (Ety/THEL), where the stem form sell- is because the Quenya syncope caused the second e to be lost and then the ancient ls became ll.
Neo-Quenya: In notes from the late 1960s, Tolkien introduced a new word nésa for “sister” (VT47/14). However, I think seler 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 meletheldi “love-sister, ✱close female friend” or ᴹQ. oselle “sworn sister”. In this sense, nésa would be limited to biological relationships, but seler would refer to sisterly (or sister-like) affection.
A noun in The Etymologies of the 1930s glossed “sister, associate”, a combination of ᴹQ. seler “sister” with the prefix o- “together” (Ety/THEL, WŌ), presumably meaning “✱sworn sister” like its male equivalent ᴹQ. otorno “sworn brother”.