But he answered and said unto him that told him, Who is my mother? and who are my brethren?
Mat 12:49 And he stretched forth his hand toward his disciples, and said, Behold my mother and my brethren! Mat 12:50 For whosoever shall do the will of my Father which is in heaven, the same is my brother, and sister, and mother. 
That is correct sis IMHO.
But imo that is speaking on a conscious level. Like these scriptures:
1Jo 3:1 Behold, what manner of love the Father hath bestowed upon us, that we should be
called the sons of God: therefore the world knoweth us not, because
it knew him not. These are the ones conscious of his love. The brethren in consciousness. The loved ones in consciousness of his love for them. And so the one able to transform because of that consciousness.
1Jo 3:14 We
know that we have passed from death unto life, because we love the brethren {the fellow conscious members}. He that loveth not [his] brother abideth in death.
If one does not love those in the consciousness of love how can one love those that are not in that consciousness? [Rhetorical question, not directed to anyone here of course]
Luk 6:32 For if ye love them which love you, what thank have ye? for sinners also love those that love them.
Luk 6:33 And if ye do good to them which do good to you, what thank have ye? for sinners also do even the same.
Luk 6:34 And if ye lend [to them] of whom ye hope to receive, what thank have ye? for sinners also lend to sinners, to receive as much again.
Luk 6:35 But love ye your enemies, and do good, and lend, hoping for nothing again; and your reward shall be great, and
ye shall be the children of the Highest: for he is kind unto the unthankful and [to] the evil. That is children by token of characteristic. Being like God in this characteristic. Being his sons by this characteristic.
Now add fulfillment to that, at least from my perspective and we really do have a situation where all are brethren. But still all are not conscious of these matters.
JMO of course
Barry