Upvote:1
I'm not sure this is a question about Christianity as such. The Oxford English Dictionary for “brother” has an entire section devoted to the religious sense of the word (section A.III), with examples from the 20th and 21st centuries, including the plural “brethren”.
In addition I noticed these secular uses of the word:
Upvote:1
Is brethren a currently used term for plural of brother (monks)?
Yes, brethren is a term that is currently used in monasteries to refer about monks, whether choir monks ad/or lay brothers.
I know of many Benedictine monasteries that still employ this term.