According to Catholicism and some evangelical denomination in order to give justifications the perpetual virginity of Mary, they affirm that the word "Brother" in the Bible is ambiguous and include cousin and relatives. But if that is the case, can we really be named brothers of Jesus? or we should be just cousins or relatives of Christ?
"Brother" is not vague, but it does have multiple subsenses. These are usually distinguished quite clearly, and Protestants who reject the perpetual virginity of Mary would say that those who argue for it have to go to great lengths to get misinterpret the verses which indicate otherwise. But that's a whole nother topic, and we don't need to get into it to answer this question.
Adoption is a major Biblical theme, and the Bible clearly says many times that we are Jesus's brothers and sisters.
Mark 3:33-35: He replied to them, “Who are my mother and my brothers?” Looking at those sitting in a circle around him, he said, “Here are my mother and my brothers! Whoever does the will of God is my brother and sister and mother.”
Romans 8:15-17: For you did not receive a spirit of slavery to fall back into fear. Instead, you received the Spirit of adoption, by whom we cry out, “Abba, Father!” The Spirit himself testifies together with our spirit that we are God’s children, and if children, also heirs—heirs of God and coheirs with Christ—if indeed we suffer with him so that we may also be glorified with him.
Romans 8:29: For those he foreknew he also predestined to be conformed to the image of his Son, so that he would be the firstborn among many brothers and sisters.
Galatians 4:4-7: When the time came to completion, God sent his Son, born of a woman, born under the law, to redeem those under the law, so that we might receive adoption as sons. And because you are sons, God sent the Spirit of his Son into our hearts, crying, “Abba, Father!” So you are no longer a slave but a son, and if a son, then God has made you an heir.
Ephesians 1:5: He predestined us to be adopted as sons through Jesus Christ for himself, according to the good pleasure of his will,
Hebrews 2:11-12: For the one who sanctifies and those who are sanctified all have one Father. That is why Jesus is not ashamed to call them brothers and sisters, saying:
I will proclaim your name to my brothers and sisters;
I will sing hymns to you in the congregation.
If the word “Brother” is ambiguous as far as the Bible is concerned, can we be named Brothers of Jesus Christ?
First of all, I would like to demonstrate what Catholicism has to say about the Brethren of the Lord:
The exact nature of the relationship between the Saviour and his "brethren"
The texts cited at the beginning of this article show beyond a doubt that there existed a real and near kinship between Jesus and His "brethren". But as "brethren" (or "brother") is applied to step-brothers as well as to brothers by blood, and in Scriptural, and Semitic use generally, is often loosely extended to all near, or even distant, relatives (Genesis 13:8, 14:14-16; Leviticus 10:4; 1 Chronicles 15:5-10, 23:21-22), the word furnishes no certain indication of the exact nature of the relationship. Some ancient heretics, like Helvidius and the Antidicomarianites, maintained that the "brethren" of Jesus were His uterine brothers the sons of Joseph and Mary. This opinion has been revived in modern times, and is now adopted by most of the Protestant exegetes. On the orthodox side two views have long been current. The majority of the Greek Fathers and Greek writers, influenced, it seems, by the legendary tales of apocryphal gospels, considered the "brethren" of the Lord as sons of St. Joseph by a first marriage. The Latins, on the contrary, with few exceptions (St. Ambrose, St. Hilary, and St. Gregory of Tours among the Fathers), hold that they were the Lord's cousins. That they were not the sons of Joseph and Mary is proved by the following reasons, leaving out of consideration the great antiquity of the belief in the perpetual virginity of Mary. It is highly significant that throughout the New Testament Mary appears as the Mother of Jesus and of Jesus alone. This is the more remarkable as she is repeatedly mentioned in connexion with her supposed sons, and, in some cases at least, it would have been quite natural to call them her sons (cf. Matthew 12:46; Mark 3:31; Luke 8:19; Acts 1:14). Again, Mary's annual pilgrimage to Jerusalem (Luke 2:41) is quite incredible, except on the supposition that she bore no other children besides Jesus. Is it likely that she could have made the journey regularly, at a time when the burden of child-bearing and the care of an increasing number of small children (she would be the mother of at least four other sons and of several daughters, cf Matthew 13:56) would be pressing heavily upon her? A further proof is the fact that at His death Jesus recommended His mother to St. John. Is not His solicitude for her in His dying hour a sign that she would be left with no one whose duty it would be to care for her? And why recommend her to an outsider if she had other sons? Since there was no estrangement between Him and His "brethren", or between them and Mary, no plausible argument is confirmed by the words with which he recommends her: ide ho uios sou, with the article before uios (son); had there been others sons, ide uios sou, without the article, would have been the proper expression.
The decisive proof, however, is that the father and mother of at least two of these "brethren" are known to us. James and Joseph, or Joses, are, as we have seen, the sons of Alpheus, or Clopas, and of Mary, the sister of Mary the Mother of Jesus, and all agree that if these are not brothers of the Saviour, the others are not. This last argument disposes also of the theory that the "brethren" of the Lord were the sons of St. Joseph by a former marriage. They are then neither the brothers nor the step-brothers of the Lord. James, Joseph, and Jude are undoubtedly His cousins. If Simon is the same as the Symeon of Hegesippus, he also is a cousin, since this writer expressly states that he was the son of Clopas the uncle of the Lord, and the latter's cousin. But whether they were cousins on their father's or mother's side, whether cousins by blood or merely by marriage, cannot be determined with certainty. Mary of Clopas is indeed called the "sister" of the Blessed Virgin (John 19:25), but it is uncertain whether "sister" here means a true sister or a sister-in-law. Hegesippus calls Clopas the brother of St. Joseph. This would favour the view that Mary of Clopas was only the sister-in-law of the Blessed Virgin, unless it be true, as stated in the manuscripts of the Peshitta version, that Joseph and Clopas married sisters. The relationship of the other "brethren" may have been more distant than that of the above named four.
The chief objection against the Catholic position is taken from Matt 1:25: "He [Joseph] knew her not till she brought forth her firstborn son"; and from Luke 2:7: "And she brought forth her firstborn son". Hence, it is argued, Mary must have born other children. "Firstborn" (prototokos), however, does not necessarily connote that other children were born afterwards. This is evident from Luke 2:23, and Ex 13:2-12 (cf. Greek text) to which Luke refers. "Opening the womb" is there given as the equivalent of "firstborn" (prototokos). An only child was thus no less "firstborn" than the first of many. Neither do the words "he knew her not till she brought forth" imply, as St. Jerome proves conclusively against Helvidius from parallel examples, that he knew her afterwards. The meaning of both expressions becomes clear, if they are considered in connexion with the virginal birth related by the two Evangelists.
Can we really be named brothers of Jesus? This would seem in a Catholic perspective that we would have to respond in the negative. However, we are all children (sons) of God by adoption.
Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will. - Ephesians 1:5
Thus it has been the traditional practice of the Church to address the faithful as Brothers and Sisters in Christ and not in the ambiguous phrase Brothers of Christ since it seems to carry connotations that we are somehow related to Jesus through an actual bloodline. A fact this is impossible to substantiate and has The DaVinci Code written all over it.
Here is just one tiny example of a Catholic traditional use of the phrase in Christ by Pope John Paul II:
Dear Brothers and Sisters in Christ,
Praised be Jesus Christ !
It gives me great pleasure to greet the Council of the International Catholic Union of the Press and other Catholic journalists with this traditional Christian salutation, because this salutation expresses an essential part of your vocation.
It's not the place to argue for the position that Mary did not have any other children than Jesus here. However, it can be confidently and unequivocally said that at the least, it was the prevailing and apparently perennial view of Christians as far as the historical record allows us to judge the matter.
For example, when a man called Helvidius in the fourth century circulated this idea, that Mary had other children, it was unknown to one of the most learned Scripture scholars and theologians of the day, Jerome:
Who, pray tell, before you appeared, was acquainted with this blasphemy? Who thought the theory worth two-pence? Against Helvidius, 18
Christians always held that the Virgin Mary never had other children: it wasn't to 'prove' such that they cited the fact that among the Hebrews "brothers" could be used of either close kin or actual siblings (as well as even friends), they only did so to make clear to those unfamiliar with the usage of the word the nature of its use with respect to Jesus.
We can't 'decide' to, or become, Christ's actual siblings, cousins or kinsmen. We can, however, become His brothers and sisters spiritually (Hebrews 2:11; Revelation 12:17; John 19:27).