And he said, Cursed be Canaan; a servant of servants shall he be unto his brethren. — KJV, Genesis 9:25
Gen 9:25 is the first occurance of brethren in the bible indicating that it means brothers.
But since jesus says in :
For whosoever shall do the will of my Father which is in heaven, the same is my brother, and sister, and mother.— KJV, Matthew 12:50
This do the will of the Father is something that comes naturally to them that have the Holy Ghost in dwelling them. Many deceivers are around.
There exists also false brethren.
How are we to deal with the brethren
We are not to offend a brother.
As we have therefore opportunity, let us do good unto all men, especially unto them who are of the household of faith. — KJV, Galatians 6:10
We should do good unto all men but especially unto the brethren.