Years ago, when I believed in the divinity of Christ, I had a difficult time with the idea of being a brother of Christ or even a son/child of God because that made me "equal with God." I once saw a tattoo on the neck of an alternative Christian which read "son of God". I honestly thought to myself "blasphemy," because I believed only Jesus can have the title of Son of God. I have since learned the error of my ways.
"You are all sons of God through faith in Christ Jesus" - Gal 3:26
Several times in John's gospel, he mentions that we are children of God. Paul also wrote to the Galatians that they are all sons of God because of their faith in Christ. Seeing Christ as God makes it hard to stomach what Paul is saying there. But Jesus himself even called us his brothers and sisters, indicating a sense of equality with his followers.
Now with a proper understanding that Christ is one of us, "a man [who] God worked through" (Acts 2:22), you begin to get a beautiful picture of our place in God's Kingdom alongside our King and high priest. We are not on a line with God, we are all subject to God. We are not God's brothers. However, we are brothers and sisters to God's Messiah, the man Christ Jesus. We have become a royal priesthood (1 Pet 2:9, Rev 1:6), the way God originally intended his children to be (Exodus 19:6); set up to serve God and reign with Christ (2 Tim 2:12) in God's coming Kingdom.
So why are we called sons of God? It appears that many groups of beings are considered sons of God: Adam was called "the son of God" (Luke 3:38), Jesus was obviously called "son of God." Many times throughout the Bible, in Genesis, Job and even Daniel, we see Angels called "sons of God." Furthermore, "those who are led by the Spirit of God are the children of God" (Rom 8:14). What is the common similarity here? They are all created directly by God and therefore are considered sons of God.
Paul mentioned in 2 Cor 5:17 "if anyone is in Christ, the new creation has come: The old has gone, the new is here." God has created something new in us, he has put his spirit within us and we have become a new creation of God as a result. Paul explains this imparting of God's spirit to those that believe, in Galatians 3:5 with his rhetorical question, "does God give you his Spirit and work miracles among you by the works of the law, or by your believing what you heard?"
"For whoever does the will of my Father in heaven is my brother and sister and mother." - Matt 12:50
A Trinitarian who see's Jesus as being God, and then considers himself God's brother, is actually in danger of elevating himself to a place where Satan supposedly elevated himself to before he fell, and fell hard. God wants us to be like Christ, to clothe ourselves with Christ, to follow his example, to listen to and obey his words (the words God gave him to say). In doing so, we will also be considered God's sons and daughters and Jesus' brothers and sisters. How glorious it is to think on that!
"Blessed are the peacemakers, for they shall be called sons of God." - Matt 5:9