Yes, it’s right to collectively refer to “brothers and sisters” as “brethren” but only in the context of addressing them as members of a religious sect, profession, or society, as when a priest or minister addresses a congregation consisting of both males and females as follows: “
Brethren, as we celebrate the Christmas holidays…” A lay person will sound so pious or exasperatingly formal addressing his or her brothers and sisters as “brethren”; can you imagine someone telling you this: “I had a nasty quarrel with my
brethren last night over our monthly allowance”? It’s obviously more sensible and idiomatic to simply address them as, well, “my brothers and sisters.” Of course, when a lawyer or someone from outside the family refers collectively to brothers and sisters in that family, the more appropriate term is “siblings,” which denotes two or more individuals having one common parent. Note how natural this statement sounds: “There was a nasty quarrel between the
siblings last night over their monthly allowance.”
“Brethren” being the plural form of the word “brother,” it does sound chauvinistic to collectively refer to brothers and sisters as “brethren.” It’s evidently for this reason that English used to have a different plural word for each, “brethren” for brothers and “sistren” for sisters, but it was a distinction that didn’t survive the test of time.
Here’s how the online
Oxford Dictionaries explains the linguistic fate of these plural words:
“Both
brethren and
sistren were used in Middle English (12th to 15th centuries) simply as the plural forms of
brother and
sister. From about 1600,
brothers began to take over from
brethren (Shakespeare used both), except when referring to fellow members of a religious community, or a society or profession. It still has this meaning today, and you may also find it used in ironic or humorous contexts (e.g. ‘our
brethren in the popular national press’).
“
Sistren, on the other hand, had fallen completely out of use by the middle of the 16th century. It has recently been revived, typically by feminist writers, with the new meaning ‘fellow women’ (e.g. ‘Lead singer Beth starts out most shows with several shout-outs to her
sistren’). This use is not yet well established in standard English.”
So there…
