Exposure to either this weekend’s General Conference or to some turn-of-the-century Mormon missionary diaries can prompt the same questions: Do the missionaries have any brothers or do they only have brethren? Where are the sistren? And What’s with all the beloving?
“Sistren” slipped out of English in the 1600s.  “Brethren” has survived to the present in religious and professional groups. Thus, as members of a religious group, when the Elders sometimes referred to male members of their order as “brethren” they conformed to the broader trend.  However, about the 1890s it seems that Mormon use of “brethren” became more unique. The graph below compares appearances of “brethren” in the Corpus of Historical American English (COHA) with its use in General Conference. 
Only one of the five traveling Elders used “brethren” in his own writing.  Mission President Duffin did so more than ten times as frequently, perhaps suggesting a top-down linguistic influence. 
The word “beloved” seems to have taken on its Mormon valence somewhat earlier than “brethren” and did not acquire the order-of-magnitude difference from popular usage until much later. 
As with “brethren,” “beloved” was not used equally by all the Elders.  Whatever the linguistic trend, however, the Elders seem to express real and deep sentiment when they wrote about “our beloved President [Duffin]” or explained that they “hated to separate with our beloved brethren [other missionaries]” or wrote reverently about “beloved” early Church leaders.
The “Southwestern States Mission” series uses the diaries of six missionaries who served in eastern Texas around 1900 to illustrate aspects of Mormon material culture, lived religion, and social History. The missionaries are Mission President Duffin and Elders Brooks, Clark, Folkman, Forsha, and Jones. The series is inspired by Ardis Parshall’s serial posting of the missionary diary of Willard Larson Jones at Keepapitchinin. Previous installment here.
 “Brethren” and “sistren” were Middle English for “brothers” and “sisters.” (Oxford Dictionaries, unsigned, “What is the female equivalent of brethren?”, accessed 2012 Mar 31). The question of “Where are the sisters in the diaries?” is more difficult and complicated. I will (hopefully) return to it in later posts.
 In subsequent posts I hope to treat subtleties of the Elders’ usage of words like brother, sister, brethren, and friend.
 The graph shows instances per million words, smoothed with a five year running average. That is, the plotted point in 1901 reflects the average of the frequencies from 1899 to 1903. Note that the HA and GC are on different scales. As expected, the formal meetings of a religious group use “brethren” far more often than a cross-section of the population. Davies, Mark. (2010-) The Corpus of Historical American English: 400 million words, 1810-2009. Davies, Mark. (2011-) The Corpus of LDS General Conference Talks: 24 million words, 1851-2010.
 Elders Brooks, Folkman, and Forsha did not use “brethren” at all. Elder Clark cites a Mormon hymn, “Farewell dear brethren we give you our Parting hand,” sometimes called “The Missionaries’ Farewell.” It was written by WW Phelps and published in Evening and Morning Star (1:5 (1832 Oct), 80, as “Farewell Hymn of the Lord’s Servants”) and was included in the 1835 Kirtland Hymnal (A Collection of Sacred Hymns, for the Church of The Latter Day Saints, 1st ed.? #50) and various other LDS hymnals through 1927. (I did not search exhaustively. It is in multiple editions of the “Manchester Hymnal” (1840-1912; “the” LDS hymnbook) and the 1927 hymnbook. It is not in the 1908 Songs of Zion published by the missions or the 1909 Deseret Sunday School Songs.) The 1889 The Latter-day Saints’ Psalmody provides a tune with harmony (#257, “Samoa,” George Careless).
 Elder Jones’ uses “brethren” 4 times out of approximately 127,000 words (31.5 words per million). President Duffin uses it 53 / 126,000 (421 WPM). Although he did use it in his own writing, many of President Duffin’s usages come in quotations from General Conference or correspondence with the First Presidency. Note also that, as mission president, President Duffin spoke in General Conference. Elder Jones later became a Stake President (which was in many ways a bigger deal then than it is now). I think there might be a hint here about the relationship between adopting institutional language and exercising institutional power (though I decline to comment in this setting on how or when divine power might influence events). On the other hand, some of the five Elders served in leadership positions in the mission and Elder Clark was about the same age as President Duffin.
 There are, of course, difficulties with equating the COHA with “popular,” but I think the equation is adequate for the general trend analysis I’m doing here.
 Instances of “beloved”: Clark, Folkman, and Forsha: 0; Brooks: 1 (18.9 WPM); Jones: 7 (55.1); Duffin: 7 (55.6).