There is no doubt that, coming out of nowhere, this idea sounds appalling. If someone were to walk up to you on the street and ask, “Hey, did you know that Jesus and Satan are brothers?” you’d run away. To understand where this idea comes from, you would have to understand God’s Plan of Salvation.
First, let’s see what the scriptures have to say about Lucifer. Isaiah said,
“How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations!
“For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:
“I will ascend above the heights of the clouds; I will be like the most High.
“Yet thou shalt be brought down to hell, to the sides of the pit” (Isaiah 14:12-15).
In Revelation, chapter 12, John said,
“And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,
“And prevailed not; neither was their place found any more in heaven.
“And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.”
In the Doctrine and Covenants it says,
“And this we saw also, and bear record, that an angel of God who was in authority in the presence of God, who rebelled against the Only Begotten Son whom the Father loved and who was in the bosom of the Father, was thrust down from the presence of God and the Son,
“And was called Perdition, for the heavens wept over him—he was Lucifer, a son of the morning.
“And we beheld, and lo, he is fallen! is fallen, even a son of the morning!
“And while we were yet in the Spirit, the Lord commanded us that we should write the vision; for we beheld Satan, that old serpent, even the devil, who rebelled against God, and sought to take the kingdom of our God and his Christ—
“Wherefore, he maketh war with the saints of God, and encompasseth them round about” (Doctrine and Covenants 76:25-29).
In the Pearl of Great Price it says,
“And I, the Lord God, spake unto Moses, saying: That Satan, whom thou hast commanded in the name of mine Only Begotten, is the same which was from the beginning, and he came before me, saying—Behold, here am I, send me, I will be thy son, and I will redeem all mankind, that one soul shall not be lost, and surely I will do it; wherefore give me thine honor.
“But, behold, my Beloved Son, which was my Beloved and Chosen from the beginning, said unto me—Father, thy will be done, and the glory be thine forever.
“Wherefore, because that Satan rebelled against me, and sought to destroy the agency of man, which I, the Lord God, had given him, and also, that I should give unto him mine own power; by the power of mine Only Begotten, I caused that he should be cast down;
“And he became Satan, yea, even the devil, the father of all lies, to deceive and to blind men, and to lead them captive at his will, even as many as would not hearken unto my voice” (Book of Moses 4:1-4).
It is readily apparent from these scriptures that there was a war in heaven in which Christ and Satan (Lucifer) fought against each other, and that there were followers of each. Satan was defeated and cast out along with his “angels” or followers. Where were all these followers, and who were they? The answer is that they were us. Christ and Satan and their followers warred in a heaven that pre-existed this earth and mortal existence. All of us lived in this heavenly spiritual realm before we were born with physical bodies on the earth. Christ was begotten of the Father, as were all of us, including Lucifer, who was a Son of the Morning. The title “Son of the Morning” could either signify that Lucifer was one of the earliest born of the spirits, or was illustrious, or both. The name Lucifer means “lightbearer” or “shining one.” Since both Christ and Lucifer were born of the Father, they were brothers in a spiritual sense, as all of us are brothers and sisters in that sense ─ we are all the sons and daughters of God. In the war in heaven, every single spirit-child of Heavenly Father took a stand. Since Lucifer and his followers were cast out, their progression stopped. They will ever remain spirits, without the opportunity to earn mortal bodies or immortal, resurrected bodies in the afterlife. All of the rest of us took Christ’s side in the war in heaven.
Since Lucifer was cast out and disowned by God, and since Satan will be cast into “outer darkness” at the end of the world, he is no longer part of the family of God, no longer a child of God. Therefore, he is bereft of relationships, and is not the brother of Christ.