If the word "Brother" is ambiguous as far as The Bible is concerned, can we be named Brothers of Jesus Christ? 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?
&quot;Brother&quot; 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.