Who are the Brethren of the LORD?

"Then His mother and brothers came to Him, and could not approach Him because of the crowd.  And it was told Him by some, who said, “Your mother and Your brothers are standing outside, desiring to see You.” But He answered and said to them, “My mother and My brothers are these who hear the word of God and do it.” Luke 8:19-21
"And He stretched out His hand toward His disciples and said, “Here are My mother and My brothers!  For whoever does the will of My Father in heaven is My brother and sister and mother.” Matthew 12:49-50

Notice how this event is recorded slightly differently by each apostle.  This is how the Spirit takes hold of the Apostles Matthew & Luke, both having seen the same event, yet recording those details in slightly different order or emphasis.  In Luke it is the hearing of the Word and the doing of it which are significant.  In Matthew it is the doing of the will of the Father.

Yet...

Since both teachings derive from apostles and from the same account, we may make additional conclusions about the account.  We may state that it is the hearing and doing of the Word of God which is the will of the Father.  And secondly it is those who demonstrate the will of the Father in hearing and doing what they read and study in the Scriptures which identify them as brethren of the Lord.

Oh how rich are the Scriptures if we would only submit ourselves to them!