What is the Greek word for brethren?

The Greek word for brethren is ἀδελφοί (adelphoi), which is the plural form of ἀδελφός (adelphos), meaning brother.

While "brethren" can also refer to a group of people united by a common cause or belief, the Greek equivalent would be ἀδελφοί (adelphoi) in this context as well.