Are Christians children or brethren of Jesus?

(I moved this answer from the comments section following the OP)

If a Christian believes Jesus of Nazareth is יהוה (YaHVeH-Jehovah/Yahweh, the tribal-national supreme deity of the OT, Judaism & Judeo-Christianity; GEN 2:4; EXO 6:3, et al.), then s/he is a spiritual child of that historical deity. If one believes Jesus is the deified supreme deity of the RCC and EO churches, then s/he is a spiritual child of that purported deity. But Jesus said: "Behold my ... brethren! For whosoever shall do the will of my Father ... the same is my brother ... (My) brothers are these who hear the word ... and do it." (MT 12:49-50; LK 8:21 KJV).

Christ is also called the bridegroom and the Church is called the Bride. Many passages confirm that. An incomplete list can be viewed at 15 Bible Verses about Jesus As A Bridegroom.

For to us a child is born, to us a son is given, and the government will be on his shoulders. And he will be called Wonderful Counselor, Mighty God, Everlasting(V) Father,(W) Prince of Peace.

(Isaiah 9)

Christ is a father, a brother, and a husband. Which is he really? I don’t see why people can’t understand that the point isn’t whether he is a physical father, or brother, or husband. It is all metaphorical. He definitely did what married people do with the whole Church. No, he didn’t! He was definitely born from the same mother and father as all of us. No, he wasn’t! He definitely married a woman and they had children together. No, he didn’t!

It is metaphorical (else he broke the laws of Leviticus 18).

Some old hymns are a poor source of obtaining one's theology from. New hymns are even worse. Modern choruses and 'praise song' can be downright atrocious. I have many worship music books, with words, and I have penciled through lots of wrong words and phrases, writing in more theologically correct ones. Then I can sing them as I play piano. Unfortunately, I often go to worship services and have to simply not sing some of the lines, or verses. Not so with the Psalms of course, but that's an even more volatile matter in some circles!

The two scriptures quoted in your question form part of the answer. Christians are called children of God in the New Testament. Jesus taught us to pray to our Father in heaven. Yet Messiah is also the "Everlasting Father"! We are may things to Jesus, and he is many things to us. We live in him, and at the point of faith when that starts to happen, he can be viewed as giving us 'new life'; we are born again of the Spirit; then we can call God "Abba, Father" (Romans chapter 8). A whole raft of metaphorical language is employed in the scriptures to give us some idea of the vastness of our new relationship to God, in Christ, through the Holy Spirit.

However, an important point is that Jesus Christ is called the only-begotten Son of God, and never "the only son". Some modern translations are misleading here, for God has more than one son! However, he only has one only-begotten Son, who was never created. That's the point. All other sons of God had a starting point in time, being created by God (and don't overlook how God the Word made everything that was made (John 1:3). This means that God the Father and God the Son (the 'Everlasting Father' of Isaiah 9:6) share that way of relating to those who are called "sons" in the New Testament.

The question deals with how we relate to Christ, and the biblical use of calling Christ 'the only begotten Son, which is in the bosom of the Father, he hath declared [God]' (John 1:18) shows how Christ relates to the Father. Theirs is an utterly unique relationship that has no starting point in time. We, however, need a starting point in time, both to begin existing physically, and to start being 'children of God', born of the Spirit. John's gospel goes into this. For instance, John wrote,

"But as many as received [Christ], to them gave he power to become the sons of God, even to them that believe on [Christ's] name, which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God." John 1:12-13 KJV

That is what brings us into a spiritual relationship with God and Christ. Then we relate to them as in a family - we are brought into the family of God, and that should open us up to understanding the astounding and unique relationship between the eternal Father and the eternal Son, and how we then relate, as children, in that spiritual family.

To ask, Are we children or brethren of Christ? is to fall short of asking the question that would give the answer. It's not "either", "or" - it's both. There is no contradiction here because this is all about a spiritual relationship that goes beyond any relationships we have as mortals on earth. It is vastly more complex and wonderful and priviledged than any human relationships, though they are often used in an illustrative way to help us start to understand.

Christians can properly be called brothers of Christ since the Lord does so Himself:

But he answering him that told him, said: Who is my mother, and who are my brethren? And stretching forth his hand towards his disciples, he said: Behold my mother and my brethren. For whosoever shall do the will of my Father, that is in heaven, he is my brother, and sister, and mother. (Matthew 12:48-50)

Further, St. Paul calls Christians sons of God.

For the Spirit himself giveth testimony to our spirit, that we are the sons of God. And if sons, heirs also; heirs indeed of God, and joint heirs with Christ: yet so, if we suffer with him, that we may be also glorified with him. (Romans 8:16-17)

Since Jesus Christ is the Son of God this means we are as brothers to him. This is futher confirmed in verse 17 where Christians are called co-heirs with Christ which clearly puts us in the same relationship with the Father as the Son.

Some might object that Jesus is the only-begotten Son of God and that therefore we cannot be his brothers.

It is true that none are equal to Christ and that he is the only Son begotten through eternal generation from the Father.

However, Christians are still adoptive sons of God, through the laver of regeneration - the sacrament of baptism, through which we receive faith, as St. Paul says:

For in Christ Jesus you are all sons of God through the faith. For as many of you as were baptized into Christ have put on Christ. (Galatians 3:26-27)

And Pope Paul III clarifies:

Pope Paul III, Council of Trent, Sess. 6, Chap. 4: “In these words there is conveyed a description of the justification of the impious, how there is a transition from that state in which a person is born as a child of the first Adam to the state of grace and of adoption as sons of God through the second Adam, Jesus Christ our savior; indeed, this transition, after the Gospel has been promulgated, cannot take place without the laver of regeneration or a desire for it, as it is written: Unless a man is born again of water and the Holy Spirit, he cannot enter the kingdom of God (John 3:5).”