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 word glossed “sister” under the root ᴹ√NŌ (Ety/NŌ), but in the entry for ᴹ√THEL Tolkien said it was “usually used of blood-kin” and its cognate was ON. wanūre “kinswoman”, which seems like a better translation. It is simply the root ᴹ√NŌ “beget” with the prefix ᴹQ. o- “together” and a feminine suffix.