“Brethren” is apparently an archaic plural of brother, dating from the 12th century. What would be the equivalent plural for sister?
Sistren
I’ve heard/seen it used many times. Sure, it isn’t as archaicly justified as brethren, but it stands to reason that there would have been little reason for a plural of sister in those times, as the only sororital (is that a word?) organizations I can think of would be nuns (assuming nuns were around then).
Cistern?
Sorority.
From the Greek soros, meaning “sister.”
Thanks for the response, this was bugging me.
rasta, I think sorority is closer to sisterhood; very similar but more of an organization than just sisters. Like fire brigade vs firefighters.
walrus:Sistren appears to be correct, dating from Chaucer.
This reminds me of a B. Kliban cartoon I saw once:
A bunch of Amish men standing around a big well. The caption? “Brethren and Cistern”
I don’t disagree that the term sistren corresponds to the term brethren, but sistren has become obsolete while brethren is still in use. So the phrase “brethren and sistren” is not current English.
DISCLAIMERS:
I don’t think it’s fair that men have their own word and women don’t.
I don’t know why it happened that way, but it did.
I’ve heard the phrase “brethren and sistren” (or “sistern”) used in public settings (almost always in church) but never as a serious address – it was always intended as mild humor.
Admittedly the term brethren is archaic – note the concurrent use of the much more common term brothers, while a similar word, children, is still the only recognized plural for child. (Nobody says childs.) But archaic is not the same as obsolete. In my church (LDS) we address each other as Brother and Sister. If there are only men present the speaker is about equally likely to use brothers as brethren. But if there are only women it’s always sisters, never sistern.
Again – I’m not arguing the answer to the question. I’m just pointing out that going around saying brethren and sistern sounds affected.
In the words of the OED
Hmmm. Most people assume “sistren” is a joke rather than a real word. But they get it, don’t they? And it is attested in Middle English, so–there’s your word.
Why not sistren'? It's grammatical, according to the same rules as brethren.’ If someone thinks it’s a jocular back-formation, then you can recite them your Chaucer, and let them know that you’re liberal arts degree is bigger:
Not fully quyk, nor fully deed they were,
But by their coat armure, and by their gear,
Heraldes knewe them wel in special,
As knights that weren of the blood royál
Of Thebes, and of sistren tuo i-born.
Coven
…Ouch! Stop throwing things…
[DISCLAIMER] I really wasn’t sure where to put this. It seems way too trivial for a GQ, yet definitely seeks a specific answer. So if the mods feel it should be elsewhere, they should do as they see fit.
[/DISCLAIMER]
I go into a public men’s room, and often notice that when guys are standing at the urinal, and they happen to be wearing shorts–gym, street, casual, whatever–it looks like
they’re holding the hem of the right leg in their free hand,
because you can see that the right leg is hiked up. Those of you who do this, why? Is it to prevent hitting your pants by mistake?
Clearly, this wasn’t the right place to post that question above!
Obviously this was supposed to be a new topic, but I was distracted and mistyped. I did try to recall it after pressing submit, by pressing “stop”, but was obviously unsucessful. Work, the curse of the posting classes.
Will some kind hearted moderator pull my fat out of the fire and delete that question of mine, before I die of embarrassment.
I’ll never be able to show my face here again!
Pluto mentioned children. We’ve discussed brethren and sistren. The obvious follow-up: Are there similar archaic plurals for mother and father? Couldn’t find anything relevant on fathren, mothren, fathern, mothern, methren.
Pure speculation, but it’s likely that similar plurals once existed when that was the common way to form plurals. They would not, I’m guessing, have been likely to survive since we don’t tend to talk about fathers and mothers in plural very often. And it’s common words that survive changes in language. A few other plural holdovers from the time when plurals were indicated by vowel changes are mouse/mice, goose/geese, foot/feet – again, words often used in plural form.
Brothers and sisters are commonly referenced in plural so “brethren” is barely hanging on while “sistren” is lost. Children are more often than not referenced in plural form and “children” is still going strong.
I haven’t really looked at this stuff in a while but a couple of other ‘n’ plurals are oxen and “kine”, an archaic plural for “cow”, with an ‘n’ and a vowel change.
Mine definitely is, because I recognized the Knight’s Tale without having to look it up… 
Sorry about the double post…hadn’t thought of this earlier, but -n plurals were fairly common in Middle English: shoon (shoes), eyen (eyes), and so forth.
pluto’s comment about more common plurals surviving is echoed almost exactly by J.R.R. Tolkien in one of the appendices to Lord of the Rings – he’s talking about why the plural of “dwarf” is usually “dwarfs” rather than “dwarves” or even something like “dwerrows.” And he’s a hell of an authority on matters of language. 
(In fact, the same is true for many forms. Think back on your foreign language studies: all the irregular verbs are words that are used all the time, like “to be” or “to do.”)