All of the sons of God are brothers. Prior to Lucifer's being cast out of Heaven for rebellion, he was one of God's children and a brother to Jesus Christ, God's Only Begotten. From the moment he was cast out, he became permanently estranged from God, was disowned by Him and lost the right to ever again be called one of God's children. Lucifier's rebellion was a reflection on him and him alone. It had absolutely no bearing on Jesus Christ's role as our Savior and Redeemer. Besides, this concept is not one invented by Mormonism. The relationship between Jesus Christ and Satan was taught anciently. In the third century, the Christian writer, Lactantius, wrote:
"Before creating the world, God produced a spirit like himself, replete with the virtues of the Father. Later He made another, in whom the mark of divine origin was erased, because this one was besmirched by the poison of jealousy and turned therefore from good to evil. He was jealous of his older brother who, remaining united with the Father, insured his affection unto himself. This being who from good became bad is called devil by the Greeks."