Charles Ellicott 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)

Whosoever shall do the will — This, then, is what Christ recognizes as the ground of a spiritual relationship. Not outward, but inward fellowship; not the mere fact of baptism, but what baptism signifies; doing the will of God, which is the essence of holiness—this is what makes a disciple as dear to the heart of Christ as was the mother whom He loved so truly.

Sister, and mother — The special mention of the sister suggests the thought that those who bore that name had joined the mother and the brothers in their attempt to interrupt the divine work.