Since the days of Jesus Christ, the members of his Church have called each other brother and sister. This was not just happenstance—it was intended.

The Savior taught us to pray to our Father who is in heaven. He spoke of himself as the Son of his Father and frequently referred to the members of the Church as the children of God. If this doesn’t indicate family relationship, I fail to understand the meaning of those terms. – William Grant Bangerter, “Relationships,” Ensign (CR) May 1976