The original usage here, “my best beloved brethren”, is unique for the text. Otherwise, we have only “my beloved brethren” (59 times in the original text), which probably explains why the 1892 RLDS edition omitted the adjective best here (although unintentionally). The 1908 RLDS edition restored the correct “my best beloved brethren” to the RLDS text. Obviously, the use of best here is a difficult reading but appears to be intended. The critical text will maintain it.
The expectedness of the phrase “my beloved brethren” once led the 1830 typesetter to replace “my brethren” with “my beloved brethren”, namely in Alma 34:28 (for discussion, see under that passage). In another instance, Oliver Cowdery initially wrote “my beloved brethren” in 𝓟 instead of “my beloved people”, the reading in 𝓞:
In Alma 24:7 Oliver may have been influenced by the subsequent use of brethren in “these our brethren the Nephites”.
Summary: Maintain the unique phraseology “my best beloved brethren” in Alma 24:12; this reading is found in the original manuscript and appears to be fully intended.