Brethren is an old plural of brother. Today you almost never use it for the men in your own family — you say brothers for that. You use brethren when you talk to, or about, the members of a group, especially a church or other religious group: a priest saying “dear brethren,” or a writer talking about “our brethren in the faith.” It can feel warm and in-group, but it also feels old, formal, and a bit Biblical. Some speakers still stretch it to other close groups, like fellow workers in the same trade, though that already sounds rather grand. Though actually it can include women in church talk, even though the word looks male. You would sound strange, or like you were joking, if you used it for two brothers at the dinner table.