Plural-only common noun; archaic plural of ‘brother’ and specialised collective term.
The pastor addressed the congregation as 'dear brethren' at the start of the service.
В начале службы пастор обратился к собранию: «дорогая братия».
Joseph was envied by his brethren for their father's favoritism.
Иосифу завидовали его братья из-за предпочтения их отца.