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 plural form of "is" is "are."