The 1830 typesetter removed the modifier other in this passage, probably because one tends to interpret the possessive pronoun his in “his other brethren” as referring to the nearest noun, Aaron, thus making the other seem unnecessary. The intended meaning, of course, is that the narrative is now returning to discuss the missionary work of Aaron (Ammon’s brother) and Ammon’s other brethren (namely, his other brothers and their missionary companions, also referred to in the text as “brethren”, as in the preface before Alma 21: “An account of the preaching of Aaron and Muloki and their brethren to the Lamanites”). With the other deleted here in Alma 22:1, his brethren now clearly refers to Aaron’s brethren. A similar kind of pronominal reference using other occurs later in the text:
Here the possessive pronoun his refers to Alma, not Helaman; the use of other forces this interpretation and is required in Alma 45:15. The critical text will restore the other in Alma 22:1.
Summary: Restore other in Alma 22:1 (“and his other brethren”) since his refers to Ammon rather than Aaron.