Note what Mormon prophets and church officials have had to say about the matter.
From their writings it is clear that they believed and taught that Jesus and Lucifer were brothers.
Bruce R. McConkie, in his work The Mortal Messiah, Vol.1, Pg.407-408 under the heading " Lucifer and the Law of Temptation" has the following to say;
"Hence, there is -- and must be -- a devil, and he is the father of lies and of wickedness. He and the fallen angels who followed him are spirit children of the Father. As Christ is the Firstborn of the Father in the spirit, so Lucifer is a son of the morning, one of those born in the morning of preexistence. He is a spirit man, a personage, an entity, comparable in form and appearance to any of the spirit children of the Eternal Father. He was the source of opposition among the spirit hosts before the world was made; he rebelled in preexistence against the Father and the Son, and he sought even then to destroy the agency of man. He and his followers were cast down to earth, and they are forever denied mortal bodies. And he, here on earth, along with all who follow him -- both his spirit followers and the mortals who hearken to his enticements -- is continuing the war that commenced in heaven."