Brothers and brethren

The Book of Mormon uses the archaic plural brethren 499 times and the modern brothers 9 times.

Brothers always refers to biological siblings, both elder and younger. Laman and Lemuel are Nephi's "brothers," and he and Sam are their "brothers," too.

my elder brothers, who were Laman, Lemuel, and Sam (1 Ne. 2:5)
I did not rebel against him like unto my [elder] brothers (1 Ne. 2:16)
thou and thy [elder] brothers should go unto the house of Laban (1 Ne. 3:4)
thy [elder] brothers murmur (1 Ne. 3:5)  

Laman and Lemuel did speak many hard words unto us, their younger brothers (1 Ne. 3:28)

Helaman's "brothers" are older than him; but Pagag, the firstborn, also has "brothers."

counsel with your elder brothers . . . be nourished by your brothers" (Alma 39:10)

the firstborn . . . was Pagag. . . . they chose all the [younger] brothers of Pagag (Ether 6:25-26)

Brethren is also used for both elder and younger male siblings. Laman and Lemuel are referred to as Nephi's "brethren" countless times, and he also refers to "Jacob and Joseph, my younger brethren" (2 Ne. 5:6). Besides this use, brethren can also refer to co-religionists, "brethren of the church" (1 Ne. 4:26, Alma 5:14). The convenient ambiguity of Nephi's words to Zoram makes it clear that the same word was used in both of these senses:

And I also spake unto him that I should carry the engravings, which were upon the plates of brass, to my [biological] elder brethren, who were without the walls. . . . And he, supposing that I spake of the brethren of the church, . . . did follow me (1 Ne. 4:24, 26).

So far, so unremarkable. It's just a word with two possible plurals, analogous to cows and kine, or cherubs, cherubim. What piqued my interest and prompted me to write this post was a handful of verses where brothers and brethren are used together, apparently with different meanings. Here is the first such instance:

And now there was a great mourning and lamentation among the people of Limhi, the widow mourning for her husband, the son and the daughter mourning for their father, and the brothers for their brethren (Mosiah 21:9).

Given the context, I assume that this means "younger brothers mourning for their elder brothers." It seems clear that we are talking about literal family relations, but if it were the same word both times in the original language, it's hard to see why it would have been translated differently. Although, as documented above, both brothers and brethren are used elsewhere in the text for both elder and younger siblings, here it may just be a convenient way of expressing "one kind of brother mourning for the other kind of brother."

The remaining instances of brothers being contrasted with brethren have to do with the four sons of Mosiah.

And the voice of the Lord came to Ammon, saying: Thou shalt not go up to the land of Nephi, for behold, the king will seek thy life; but thou shalt go to the land of Middoni; for behold, thy brother Aaron, and also Muloki and Ammah are in prison. Now it came to pass that when Ammon had heard this, he said unto Lamoni: Behold, my brother and brethren are in prison at Middoni, and I go that I may deliver them (Alma 20:2-3).

Ammon and Aaron are biological siblings, two of the four sons of Mosiah. We don't know who exactly Muloki and Ammah are (this is the first of only two times they are mentioned), but they are apparently either kinsmen in a broader sense (cousins or such) or biologically unrelated "brethren of the church."

Muloki and Ammah were not the only "brethren" preaching alongside the four brothers.

[Ammon, after having parted from his biological brothers,] departed out of their synagogue, and came over to a village which was called Ani-Anti, and there he found Muloki preaching the word unto them; and also Ammah and his brethren. . . . Aaron and a certain number of his brethren were taken and cast into prison, and the remainder of them fled out of the land of Middoni unto the regions round about. And those who were cast into prison suffered many things, and they were delivered by the hand of Lamoni and Ammon, and they were fed and clothed (Alma 21:11, 13-14).

The "brethren" of Ammah are apparently actual siblings or kinsmen, since if they were co-religionists they would not have been specifically Ammah's brethren. In Alma 20, the Lord tells Ammon that Aaron, Muloki, and Ammah are in prison, but here it is "Aaron and a certain number of his brethren" -- which would be an unlikely way of phrasing it if the "certain number" were two and if they were the two people who had just been mentioned by name. So it was apparently more than just those three who were in prison, but the Lord only mentioned the three to Ammon. This perhaps suggests that Muloki and Ammah may have been Ammon and Aaron's "brethren" in a different sense than the other prisoners. There is a further hint of this a couple of chapters later:

Behold, now it came to pass that the king of the Lamanites sent a proclamation among all his people, that they should not lay their hands on Ammon, or Aaron, or Omner, or Himni, nor either of their brethren who should go forth preaching the word of God, in whatsoever place they should be, in any part of their land (Alma 23:1).

This certainly makes it sound as if Ammon, Aaron, Omner, and Himni (the four biological brothers) had only two "brethren" -- who would presumably be Muloki and Ammah. However, we can't say for sure that this is the intended meaning. In Websters' 1828 dictionary (describing American English as used at the time the Book of Mormon was translated), these are the first two definitions of either:

1. One or another of any number. Here are ten oranges; take either orange of the whole number, or take either of them. In the last phrase, either stands as a pronoun or substitute.

2. One of two. This sense is included in the foregoing.

So the first definition is one "of any number." And it does make more sense that the king would forbid the Lamanites to lay hands on any and all Nephite preachers rather than singling out six individuals. Most other instances of either in the Book of Mormon clearly mean "one of two"; however, there is one unambiguous counterexample:

And now, behold, the Lamanites could not retreat either way, neither on the north, nor on the south, nor on the east, nor on the west, for they were surrounded on every hand by the Nephites (Hel. 1:31).

Here, "either way" clearly means any of four ways. Taking this into account, I think it is relatively unlikely that Alma 23:1 is singling out Muloki and Ammah.

Here is the final verse to distinguish brothers from brethren:

And now behold, Ammon, and Aaron, and Omner, and Himni, and their brethren did rejoice exceedingly . . . . And now, these are the words of Ammon to his brethren, which say thus: My brothers and my brethren, behold I say unto you, how great reason have we to rejoice (Alma 25:17, 26:1)

Here I think it is tolerably clear that Ammon's "brothers" are his actual siblings, Aaron, Omner, and Himni, while his "brethren" could be either specifically Muloki and Ammah or else his "brethren of the church" in general.