From the CRI, Christian Research Institute- because Truth Matters

Does Mormonism REALLY teach that Jesus is the spirit brother of Satan?

To begin with, according to official Mormon teaching, Jesus Christ is the first spirit child conceived and begotten by Heavenly Father and one of Heavenly Father’s many wives (commonly referred to as “Heavenly Mother”). Just as Heavenly Father before him progressed to godhood, so Jesus progressed through obedience to the status of a god (prior to his incarnation on earth). In the words of the late Mormon Apostle and General Authority Bruce McConkie, Jesus Christ through obedience and devotion “attained that pinnacle of intelligence which ranked him as a God.” As such, according to LDS authorities, Jesus is not to be worshiped or prayed to as one would worship or pray to Heavenly Father.
Furthermore, Mormons teach that Heavenly Father subsequently had other spirit children. We ourselves are thought to be spirit children of Father God and Mother God. As such, Mormons refer to Jesus as our “Elder brother.” As the official LDS teacher’s manual Gospel Principles explains, “We needed a Savior to pay for our sins and teach us how to return to our Heavenly Father. Our Father said, ‘Whom shall I send?’ (Abraham 3:27). Two of our brothers offered to help. Our oldest brother, Jesus Christ, who was then called Jehovah, said, ‘Here am I, send me’” (emphasis added).
Finally, it stands to reason that if Jesus is the first spirit child conceived and begotten by Heavenly Father; and if Heavenly Father and Mother subsequently conceived other spirit children including Satan; then Jesus and Satan logically are spirit brothers. While LDS spokespersons sometimes obfuscate this fundamental Mormon teaching, apostles of the Mormon Church and current official LDS publications clearly affirm it. As explained by the Mormon publication Ensign, “On first hearing, the doctrine that Lucifer and our Lord, Jesus Christ, are brothers may seem surprising to some— especially to those unacquainted with latter-day revelations. But both the scriptures and the prophets affirm that Jesus Christ and Lucifer are indeed offspring of our Heavenly Father and, therefore, spirit brothers.”
In sharp contrast to Mormon Christology, the biblical witness is clear and convincing: Jesus Christ is the eternal Creator God (John 1; Colossians 1; Hebrews 1; Revelation 1). Paul explicitly teaches that Jesus is the creator of all, including the angelic realm to which Satan belongs (Colossians 1:15–16; cf. John 1:3). Jesus is thus Satan’s creator, not his spirit brother.
For further study, see “Is Mormonism Christian?” p. 253; and “Is the Book of Mormon credible?” p. 257; and Hank Hanegraaff, “The Mormon Mirage: Seeing Through the Illusion of Mainstream Mormonism” (Charlotte: Christian Research Institute, 2008).
Colossians 1:15–16 “He is the image of the invisible God, the firstborn over creation. For by him all things were created: things in heaven and on earth, visible and invisible, whether thrones or powers or rulers or authorities; all things were created by him and for him.”