Christ said that we are His brothers.
Matthew 12:48-50 ESV
But he replied to the man who told him, “Who is my mother, and who are my brothers?” And stretching out his hand toward his disciples, he said, “Here are my mother and my brothers! For whoever does the will of my Father in heaven is my brother and sister and mother.”
Romans 8:15-17 ESV
For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.
If we are God's adopted sons, and joint heirs with Jesus, then we are indeed "brothers". But as well, you are correct that Christ is our LORD and our GOD! How awesome and amazing is it that we are both surrendered to our Lord and God and yet ADOPTED as family and given the right to be joint heirs for all eternity. We cannot even fathom such amazing grace!