Matthew 12:50

For whosoever shall do the will of my Father who is in heaven, he is my brother, and sister, and mother.

Relationship to Christ does not depend on fleshly kinship but on obedience to God's will. As John's gospel has it, "As many as received him, to them gave he the right to become children of God, even to them that believe on his name: who were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God" (John 1:12,13).