Not to subtract from what others have said, but to add to it by providing more historical citations in context, here is what the OED has to say about sistren and sistern. For some reason, these were uncommonly difficult to search for. All are taken from entries that have not been updated since the OED2.
The first form, sistren, can be found in the citations for brother n., dean n.1, sect n.1, and sister n. Here’s the one from brother n. sense 3a:
- c1449 R. Pecock Repressor (1860) 63 Thi Cristen britheren and sistren.
And here from sense 9b of dean n.1:
- 1389 in T. Smith & L. T. Smith Eng. Gilds (1870) 46 On Dene, for to warnyn alle þe gild breþren and sistren.
Here from sect n.1 sense 1b:
- 1393 Langland Piers Plowman C. xvii. 293 Þoȝ men soȝt al sectes [v.r. þe sektis] of sistren & of breþeren.
Finally and most profitably from sister, n., sense 1b, which is given as:
- b. In older forms of the plural.
In Old English the plural had either the same forms as the singular, or appears as sweostra, -tru, etc. These subsequently assumed the pl. -n of weak nouns, and gave the common Middle English forms sustren, sostren, sistren, etc. (cf. brethren (see brother n.)). In general literary use these were finally discarded about 1550 in favour of the pl. in -s, which is found as early as c1200.
And under which appear these relevant citations, all from the β citations of this sense. I also provide older citations with alternate forms to show the (somewhat free) variation seen over time.
- c950 Lindisf. Gosp. Mark vi. 3 Ahne suoestro [c1000 swustra, c1160 swustre] his her mið usic sint?
- c1100 Anglo-Saxon Chron. (MS. D) ann. 1067, Mid his modor & his twam sweostran.
- [c1160 Hatton Gosp. Mark iii. 35 Se is min moder & min broðer & mine swustren.]
- c1290 S. Eng. Leg. I. 435 Þat þou sum-ȝware þine sostrene do in-to ane nonnerie.
- 1297 R. Gloucester’s Chron. (Rolls) 7560 His moder & is sostren tuo mid him sone he nom.
- c1507 in T. Stapleton Plumpton Corr. (1839) 202, I recommend me to you,‥and to all my brethren and sistren.
- 1532 (1385) Usk’s Test. Loue in Wks. G. Chaucer III. f. ccclv, As susterne in vnite they accorden.
- 1553 T. Wilson Arte of Rhetorique 30 b, You have other parentes, other brethren, sisterne, and nephewes.
- 1580 in J. Raine Wills & Inventories N. Counties Eng. (1835) I. 432, I will that all the goodes be devyded equallye amongeste my Brethren and systeringe childringe.
- 1859 J. R. Bartlett Dict. Americanisms (ed. 2), Sistern, for sisters. A vulgar pronunciation sometimes heard from uneducated preachers at the West.
Notice how in the final citation, from American linguist John Russell Bartlett (1808–1886), that sistern is by then considered vulgar and uneducated.
The form occurs again in this citation from sense 3a:
- 1482 in Eng. Hist. Rev. XXV. 122 Ye kepar of oure ye sistrenes librarie.
Then several times again in citations from sense 3b, the second being the one mentioned by Barrie:
- c1449 R. Pecock Repressor (1860) 63 Therbi [thou] enhauncidist thi silf aboue thi Cristen britheren and sistren.
- 1533 T. More Apol. iv, in Wks. 849/2 Now was this word taken vp, & walked about abrode among the brethren & sistern.
- a1849 H. Coleridge Ess. & Marginalia (1851) I. 375 We united brethren and sisteren of the three kingdoms.
- 1861 N. A. Woods Tour Prince of Wales in Canada 261 The cortège had to be eked out with the Temperance Brethren and Sistren.
Here is the proper citation, since I have quoted so much from the entry:
Second edition, 1989; online version December 2011.
http://www.oed.com/view/Entry/180434; accessed 04 February 2012. Earlier version first published in New English Dictionary, 1911.
On sisterns & cisterns
The second form, sistern, I have already given as occurring under sister n. It also occurs in the first citation for child, n. sense 2b, where it appears under the spelling sisterne, with a final e:
- 1382 Bible (Wycliffite, E.V.) Gen. xxxvii. 30 Ruben turned aȝen to the sisterne, fonde not the child [i.e. Joseph æt. 17].
Note that our modern word cistern was during Middle English sometimes spelled systerne or sisterne. Here are two citation for such, from cistern, n.:
- 1382 Bible (Wycliffite, E.V.) Gen. xxxvii. 22 Throw ȝe him into the olde sisterne, that is in wildernes.
- 1481–90 Howard Househ. Bks. (1841) 353 Paied to a carpenter for mendynge of a systern iij.d.
It is also found in citations for winch, n.1:
- c1440 Pallad. on Husb. i. 426 In stede of welle or wenche [orig. fons‥aut puteus] haue a sisterne.
- c1440 Pallad. on Husb. iii. 894 The water cleer Of cisterne or of wynche.
So the word has not been used for quite sometime. The only publicly visible scheduled update is under sister, and refers to that word’s use in the gay community; the update was entered in 1997.
However, the question on What is the female equivalent of brethren? from Oxford Dictionaries’ online site suggests that though having long ago fallen into disuse, that it has been recently revived:
Sistren, on the other hand, had fallen completely out of use by the middle of the 16th century. It has recently been revived, typically by feminist writers, with the new meaning ‘fellow women’ (e.g. Lead singer Beth starts out most shows with several shout-outs to her sistren). This use is not yet well established in standard English.
I suggest that were one so inclined to do so, that one could best help re-establish the word in standard English in the easiest way possible: simply by using it.