Oliver Cowdery, the scribe in 𝓞, at first expected the plural brothers since this is the third brother that Alma has addressed. Oliver initially wrote the first two instances as brothers (with the b capitalized); then in each case, he immediately erased the plural s. The following singular pronouns, his and he, show that the singular is intended. Thus when Oliver finally got to the third instance of brother (in verse 2), he wrote it in the singular without error. And only then do we finally realize that Alma is speaking about Shiblon as the other brother, not Helaman, since Shiblon was the brother who was with Corianton “among the people of the Zoramites”. (Alma 31:7 explains that Helaman did not go on the mission to the Zoramites.) Moreover, Alma’s words to Shiblon, especially in Alma 38:10–14, could be interpreted, perhaps incorrectly, as applying to Shiblon personally (for instance, “see that ye are not lifted up unto pride / yea see that ye do not boast in your own wisdom”), with the result that here at the beginning of chapter 39 one might think that Alma was referring to Helaman, not Shiblon, as the unnamed brother. Yet it is Shiblon that Alma praises for his steadiness, faithfulness, and diligence: “because of your steadiness and your faithfulness unto God … because of thy faithfulness and thy diligence and thy patience and thy long-suffering among the people of the Zoramites” (Alma 38:2–3). And finally, as Alma himself observes here in Alma 39:1 (“I have somewhat more to say unto thee than what I said unto thy brother”), his message to Shiblon was quite short, while his message to Helaman was almost as long as the one to Corianton (we get the following number of pages in the 1830 edition for each of the three discourses: 7.0 for Helaman, 1.5 for Shiblon, and 8.2 for Corianton).
So by the middle of verse 2, it would have become clear to Oliver Cowdery that Alma is recommending Shiblon to Corianton. Of course, Alma could have recommended both brothers; in fact, he does precisely that later on in this chapter:
The original manuscript is not extant for the first occurrence of brothers in Alma 39:10, although it probably read that way (but with a capitalized B). In the printer’s manuscript, Oliver initially wrote the singular brother for the first occurrence in verse 10, probably because he had gotten used to the singular brother in verses 1–2. Here in verse 10, Oliver virtually immediately corrected brother to brothers in 𝓟 by overwriting the final r with rs. And for the second occurrence of the word in verse 10, the text reads your brothers in both manuscripts and without variation. Alma is indeed referring to both older brothers in verse 10.
There is a similar switching in number for brother(s) at the beginning of 1 Nephi, where the text first refers to Nephi and Sam, then to Nephi alone:
Here Oliver Cowdery once more had difficulty writing the correct number (and understandably so). For discussion, see under 1 Nephi 3:28, 29. Ultimately, Oliver was able to get the number correct in 1 Nephi 3:28–29 as well as here in Alma 39:1–2 and in Alma 39:10.
Summary: Maintain the use of the singular brother in Alma 39:1–2 and the use of the plural brothers in Alma 39:10; despite the tendency to mix up the grammatical number for brother, Oliver Cowdery was finally able to correctly write down the appropriate singular and plural in Alma 39.