Brethren is plural. It means "brothers".
The singular form would be "brother".
The possessive form for the plural noun brethren is brethren's.Example: Our brethren's fund raising efforts support our services for seniors.
"Sisters" is appropriate anyway, but the alternate "sistren" is sometimes used.
One example of a plural word that ends in "ren" like "children" is "brethren," which is the plural form of "brother." Another example is "oxen," the plural form of "ox." These words come from Old English and have retained their unique plural forms over time.
Brothers and Brethren.
The singular form of brethren is "brother."
"Brethren" is a plural word for "Brother", akin to "Brothers". It is perhaps a more archaic way to signify a plurality of men who are considered a brother.
The plural of bràthair (brother) is bràithrean. (This would not be for the surname Brothers.)
Brethren is the plural for brother. George and his brethren went to town for supplies. Or more commonly stated; George and his brothers went to town for supplies.
There is no plural form. Do and Do not are verbs
The plural form of him, her, or it is them. (objective pronouns)
Bridges is the plural form of bridge.
The word 'brethren' is an archaic (historical) plural of our word 'brother'. According to www.etymonline.com, this older form of the plural was commonly used between the 13th and 17th centuries, before the modern form 'brothers' began to be used. 'Brethren' is still an English word, but tends to be reserved for religious usage (or intentionally archaic uses). The -en ending for plurals in English is a Middle English form (the form of our language predominant between about the 11th and 15th centuries) based on an Old English (the oldest form of English, spoken 5th-11th centuries) weak declension - for this reason you'll never see a native word in common usage which has obviously been introduced after that time pluralised with -en ('computeren', for instance, or 'helicopteren' - although there is an example in 'Leet-speak' of a slang term for computer, 'box', pluralised to 'boxen', this is a humorous usage). Other examples of words which use the -en plural form are 'children' and 'oxen'.