Mormons don't believe Jesus and Lucifer are brothers. They believe Jesus and Lucifer and other spirits are all spirit brothers.
Whatever that means.
You’re right. But, I know, whatever that means.
“’Who will redeem the earth, who will go forth and make the sacrifice for the earth and all things it contains?’ The eldest son said: ‘Here am I;’ but he did not say ‘send me.’ But the second one, which was ‘Lucifer, son of the morning,’ said, ‘Lord, here am I, send me, I will redeem every son and daughter of Adam and Eve that lives on the earth,...”
Journal of Discourses, vol 13: page 282
(A standard work of the LDS church)
The Eldest Son in the above is Jesus. I can send you another passage that expounds on this. It sure sounds like brothers to me.
You have to realize that many of the terms used by Christians are also used by Mormons, but with different meanings. For example, immortality and eternal life are separate things in Mormon theology, but unless you realize that, it's easy to misunderstand a Mormon and think that what he's telling you isn't so different from orthodox Christianity.
So...here we go... There's a Heavenly Father (Elohim), but also a Heavenly Mother. They had many spirit children, but Jesus was the first and highest of them. Lucifer wanted to be the Saviour of us on Earth, but wanted to save everyone...and got upset when Jesus was made Savior for Earth (with his plan of free agency--allowing man to choose to follow a path of salvation or not). Also, however, note that salvation isn't a mere choice to follow Jesus, but it is a collection faith and works...without the right combo of "merit badges," you can't get to the highest level of heaven and get stuck with other poor souls who never got married, etc.
As for the little baby in the manger, well...God came to Mary in the flesh and impregnated her, yielding Jesus. As the LDS apostles have explained, God (Heavenly Father) is the literal father of both the spiritual Jesus and the incarnate Jesus. Oh, and in case you're wondering how Mary was still a "virgin" after getting it on with God the Father...well, you see, "virgin" means that she never had sex with a mortal. That night with the Heavenly Father doesn't count, of course!
Oh, and just to clarify, Jesus was most definitely not begotten of The Holy Spirit (though he played some part...just don't ask me to explain...)
This is from LDS teaching, as well as Joseph Smith, Brigham Young, etc. And if you want to get into the twists and turns of what is believed about Old Testament terminology versus New Testament meanings of the same words, your head will start spinning. You have to keep track of when Jehovah refers to Jesus instead of the Father, etc.
So what.
That’s a nice-looking dead horse you’re flogging right there.