Both the one who makes men holy and those who are made holy are of the same family. So Jesus is not ashamed to call them brothers.
My friend Tim nailed it again when he wrote the following in response to the verse above:
"Christ can call me ‘brother’ because I am holy through Him. No other reason at all... none."
I love listening to people testify rightly with regard to who they are and who God is and how much greater and good and worthy of praise He becomes in their minds and hearts in dwelling upon these things.