Brethren (81) (adelphotes from adelphós = brother) is more literally "the brotherhood" which in Greek is a so-called collective singular, which indicates more strongly than the word "brothers" the oneness of those subjected to these sufferings. The only other use of adelphotes is found in 1Pe 2:17-note. Adelphos is derived from the copulative prefix (one that joins together words and expressing addition of their meaning) "a" and delphus “the womb” which gives the meaning of “one born from the same womb.” Brotherhood is the quality or state of being brothers, sharing a common bond, and in this context a common "birth", the new birth, all of one family ("children of God" Jn 1:12, 1Jn 3:1-note, 1Jn 3:2-note). BDAG (1) a group of fellow-believers, a fellowship...of the Christian community, whose members are adelphoi and adelphai 1 Pt 5:9; 1 Clemen 2:4...1 Pt 2:17...(2) mutual affection such as brothers or sisters have for one another, familial affection Norman Hilyer Peter’s choice of the collective term emphasizes the common bond of experience shared by all Christian believers, even those unknown to the readers. (New International Biblical Commentary) The Anchor Yale Bible Dictionary... Adelphos and the feminine form, adelphÄ", are used first to speak of physical relationships, but approximately half of the occurrences in the NT use the figuratively/spiritually, primarily to speak of relationships between the people of Israel or between Christians.