Craig, in the verse JFK posted about who our brothers/brethren are "whoever does the will of My Father," we can see in Luke that Jesus actually goes a little further in that comment to say "these who hear the word of God and DO IT."
Luke 8:21 But He answered and said to them, "My mother and My brothers are these who hear the word of God
and do it."
Matt 7 show those that "he who does the will of My Father," are the only ones to enter the kingdom. That narrows who it is that do/obey the Father's will down considerably.
Mat 7:21 "Not everyone who says to Me, "Lord, Lord,' shall enter the kingdom of heaven,
but he who does the will of My Father in heaven.
v. 22 Many will say to Me in that day, "Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?'
v. 23 And then I will declare to them, "I never knew you; depart from Me, you who practice lawlessness!'
There is very much in doing/obeying the Father's will as Ray had much to say about in 'What is the Father's will?'
http://forums.bible-truths.com/index.php/topic,3720.0.html Hebrews further explains it is those being prepared for salvation that are brothers or "brethren."
Heb 2:10 For it was fitting for Him, for whom are all things and by whom are all things, in bringing many sons to glory, to make the captain of their salvation perfect through sufferings.
v. 11 For both He who sanctifies and those who are being sanctified are all of one, f
or which reason He is not ashamed to call them brethren,
v. 12 saying:
"I will declare Your name to My brethren;
In the midst of the assembly I will sing praise to You."
mercy, peace and love
Kat