'Brethren' in the Bible

Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning.

Marvel not, my brethren, if the world hate you.

We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death.

Hereby perceive we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren.