John Gill Commentary Matthew 12:50

"For whosoever shall do the will of my Father who is in heaven, he is my brother, and sister, and mother." — Matthew 12:50 (ASV)

For whoever does the will of my Father
This is not to be understood of a perfect obedience to the will of God, revealed in his righteous law; for since this cannot be performed by any mere man, no one could be in such a spiritual relation to Christ: but of the obedience of faith to the will of God, revealed in the Gospel; which is to believe in Christ, and have everlasting life; see (John 6:40) . This is the will of Christ's Father, which is in heaven ,
and which is good news from heaven, to sinners on earth; and which Christ came down from heaven to do, and to declare to the children of men: such as "hear the word of God and do it", as Luke says, (Luke 8:21) that is, hear the Gospel, understand and believe it, and become obedient to the faith of it; these are in this near manner related to Christ, evidentially and openly, as well as those who were now present:

the same is my brother, and sister, and mother ;
as dear to me, as such are to those, to whom they were thus related in the flesh: and these natural relations serve to convey some ideas of that relation, union, nearness, and communion, there are between Christ and his people; all these relative characters may be observed in the book of Solomon's Song, to which our Lord may be reasonably thought to have respect; see (Song of Solomon 3:11) (Song of Solomon 4:9Song of Solomon 4:10Song of Solomon 4:12) (Song of Solomon 5:1Song of Solomon 5:2) (8:1) .