"Mormons teach that Jesus is the brother of Lucifer. They believe that the spirits of Jesus and Lucifer were first born to Heavenly Father in heaven that Mormons call the pre-existence. According to Mormon Scripture, Jesus was the first one born and Lucifer was the second one and then all the other spirits of humans and angels were born. "
https://www.4witness.org/mg-22-do-mormons-believe-jesus-and-lucifer-are-brothers/