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
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.