From a non-Mormon POV, they both were created by God the Father and pre-existed for some time, Jesus certainly/probably from the beginning. Now I read in my bible that Lucifer had some sort of special status before he rebelled and took 1/3 of the angels with him. So what is wrong with thinking of them of at least spiritual brothers? Cain and Abel were brothers. One ended up murdered by the other. Jesus, I guess I'm supposed to believe he was the only begotten son, I don't really know what that means theologically (please don't try to explain it). I believe Jesus had a creator role. He was foreordained to be the savior. Lucifer was the Morning Star whatever that is.
I'm going way out on a limb here :-) and state emphatically that I do not believe any Mormon thinks Jesus and Lucifer were on good terms after the fall. They became enemies after that. Before that? I doubt it if they were both doing the will of the Father.
Now this is just my personal opinion, but I believe that Lucifer can muck up creation but I can't cite a bible passage. I just believe he can manipulate life to a certain extent, who, what, and how, don't ask, I don't know. He just can't create it.
So what is the big deal about it really? Probably that the remark was intended to be derogatory and inflammatory. For that reason he should apologize. If the apology was heartfelt and sincere. How do we know? We don't. At least it was a reaching of sorts across the great divide.
What Every Mormon Should Know About Astronomy
The Savior said, "I Jesus...am the root and the offspring of David, and the bright and morning star" (Rev. 22:16). By this He was almost certainly acknowledging the sign or symbol of the planet Venus. Because it is between the earth and the sun, it always appears near the sun in our sky and is only visible near dusk or dawn. Thus, to us it is an evening and morning star. According to Native American tradition, the cycle of Venus is like the life of the white, bearded god who visited them in the first century A.D. Their legends are so similar to the account of the Savior visiting the Nephites, that there seems little doubt that they refer to Jesus Christ.(3) Venus is born as an evening star rising in the west, dim but growing brighter every day. Several months later, just when it nears its brightest point, it dies, plunging quickly into the earth and disappearing as an evening star. For several days it remains underground, fighting evil forces, until it conquers death and resurrects in the east as a radiant morning star. The parallels with the life, death, and resurrection of the Savior are obvious.
I think the pattern like Jesus and Lucifer, Cain and Able, or Moese the Pharaoh, Jacob and Esau etc.
The Book of Mormon scriptures says
11 For it must needs be, that there is an opposition in all things. If not so, my first-born in the wilderness, righteousness could not be brought to pass, neither wickedness, neither holiness nor misery, neither good nor bad. Wherefore, all things must needs be a compound in one; wherefore, if it should be one body it must needs remain as dead, having no life neither death, nor corruption nor incorruption, happiness nor misery, neither sense nor insensibility
I believe Jesus had a creator role.
1 John 1
Christ is the Word of God—He created all things and was made flesh
14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. (Jesus is the only one begotten by the Father in the Flesh, the rest of us came into the earth by a mortal father.)
When we see this in prespective the role that our Savior plays and the opposition Lucifer/Satan and his agenda which can befound throuh out the scriptures with this knowledge it gives us a deeper insight as to what in going on in the world.