The Old Testament shows that brother and brethren had a wide range of meaning and could refer to any male relative other than father or grandfather.
Below, the word brethren is being used to describe the relationship between Abraham and Lot.
Genesis 13:8 And Abram said unto Lot, Let there be no strife, I pray thee, between me and thee, and between my herdmen and thy herdmen; for we be brethren.
But Abraham was Lot's uncle, as shown here.
Genesis 14:12 And they took Lot, Abram's brother's son, who dwelt in Sodom, and his goods, and departed.
Below, Laban calls Jacob his brother. Laban is actually Jacob's uncle.
Genesis 29:15 And Laban said unto Jacob, Because thou art my brother, shouldest thou therefore serve me for nought? tell me, what shall thy wages be?
Also to show that brethren can also mean cousin.
1 Chronicles 23:21 The sons of Merari; Mahli, and Mushi. The sons of Mahli; Eleazar, and Kish. 22 And Eleazar died, and had no sons, but daughters: and their brethren the sons of Kish took them.
Eleazar and Kish were brothers. Eleazar's daughters married the sons of Kish, his brother.
For other OT examples, see (Deut. 23:7; Neh. 5:7; Jer. 34:9), (2 Kgs. 10:13–14, the forty-two "brethren" of King Azariah).
Since the early Church, theologians have interpreted Mary’s confusion about how she would conceive Jesus, as an indication she had taken a vow of virginity. It was very common that young girls raised in the temple and consecrated to God take a vow of virginity. At maturity they could no longer live in the temple because their menstrual cycle made them "unclean". (They had primitive ideas back then.) The girl would be betrothed to an older man for her protection. This older man would live with her as a brother or uncle and ensure she did not break her vow and remained pure.
Luke 1:34 Then said Mary unto the angel, "How shall this be, seeing I know not a man?
If she had planned to have children that would have been a silly question. She said "I know not a man", so she knew where babies came from. She would have thought or said something like "yeah, of course I am going to have a child, that's why I got married." What had her confused was her vow of virginity. How could she, a virgin, conceive a child.
OK, now to Jesus. St. Paul writes in 1 Corinthians 15:6 “After that, He was seen of above five hundred brethren at once”. Does that mean Mary gave birth to over 500 children? Of course not!
Matthew 12:49 And He stretched forth His hand toward his disciples, and said, "Behold my mother and my brethren! 50 For whosoever shall do the will of my Father which is in heaven, the same is my brother, and sister, and mother."
Above, Jesus uses the word brethren to describe His disciples and brother as whoever does the will of His father. Jesus, Himself is giving us the Biblical definition of brother.
At the Resurrection Jesus tells Mary Magdalene to go tell His brethren:
John 20:17 Jesus saith unto her, "Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God.”
Who does Mary go to? His disciples.
John 20:18 Mary Magdalene came and told the disciples that she had seen the Lord, and that he had spoken these things unto her.
Don't you think that if Jesus had biological brothers that with such amazing news, Mary would tell them first, not the disciples? Jesus Himself clearly meant "brethren" as people close to him, not siblings.
John 19:25 Now there stood by the cross of Jesus His mother, and His mother's sister, Mary the wife of Cleophas, and Mary Magdalene.
Compare that with:
Matthew 27:56 Among which was Mary Magdalene, and Mary the mother of James and Joses, and the mother of Zebedee's children.
This shows that this James and Joses were sons of Mary the wife of Cleophas. They were Jesus's cousins. Another James, below is the son of Alphaeus.
Acts 1:13 And when they were come in, they went up into an upper room, where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James.
Above James is the brother of Simon and Judas. So, this shows that the word brother mentioned below does not indicate biological brothers.
Mark 6:3 "Is not this the carpenter, the son of Mary, the brother of James, and Joses, and of Juda, and Simon? and are not his sisters here with us?" And they were offended at him.
Also, the above calls Jesus "the son of Mary", singular, not 'a' son, meaning there are others.
When He was dying on the Cross, Jesus entrusted his mother to the apostle John.
John 19:26 When Jesus therefore saw His mother, and the disciple standing by, whom He loved, He saith unto His mother, "Woman, behold thy son!" 27 Then saith He to the disciple, "Behold thy mother!" And from that hour that disciple took her unto his own home.
Now if the previously mentioned "brethren": James, Joses, Simon, Juda, etc. were really His biological brothers, Jesus would have entrusted Mary to the oldest of them. To do otherwise would be a slap in the face of His, if they actually existed, "blood" brothers.
A historical document called The Protoevangelium of James, written around A.D. 150, while not Biblically inspired, speaks of Mary as a consecrated virgin since her youth, and of St. Joseph as an elderly widower with children who was chosen to be Mary’s spouse for the purposes of guarding and protecting her while respecting her vow of virginity.
There are more examples that show there is no evidence that Mary had other children. Trying to show so called "proof text" because it uses the words brother or brethren is fruitless as I have shown. There are many more that I did not show. But you can find them if you really want to.
Bonus section:
Famous Protestant leaders that believed Mary was a virgin before, during, and after the birth of Christ.
Martin Luther. "Christ, our Savior, was the real and natural fruit of Mary’s virginal womb . . . This was without the cooperation of a man, and she remained a virgin after that. […] Christ . . . was the only Son of Mary, and the Virgin Mary bore no children besides Him . . . I am inclined to agree with those who declare that ‘brothers’ really mean ‘cousins’ here, for Holy Writ and the Jews always call cousins brothers."
(Sermons on John)
John Calvin. John Calvin started what is now known as the Reformed theological tradition, which reject many key Catholic dogmas. Though he argued that those who reject Mary’s perpetual virginity based on Scriptural passages that mention Jesus’ “brothers and sisters” show “excessive ignorance.”
Huldrych Zwingli. Zwingli was a Swiss Reformer during the lifetime of Martin Luther. On the topic of Mary’s perpetual virgin, he wrote: "I firmly believe that Mary, according to the words of the gospel as a pure Virgin brought forth for us the Son of God and in childbirth and after childbirth forever remained a pure, intact Virgin."
Thomas Cranmer. Cranmer was the Archbishop of Canterbury during Henry VIII’s schism from Rome and thereafter and was a major figure in building Anglicanism. Yet he and other major Anglican leaders maintained the perpetual virginity of Mary “on the basis of ancient Christian authority.”
John Wesley. Wesley’s teachings and ministry led to the worldwide Methodist movement. In his Letter to a Roman Catholic, he wrote: "I believe that He [Jesus]... being conceived by the singular operation of the Holy Ghost, and born of the blessed Virgin Mary, who, as well after as before she brought Him forth, continued a pure and unspotted virgin."
P.S.
Something that really baffles me, is how so many people are fixated on Mary being sexually active.
+JMJ+