From a historical standpoint, in regards to the perpetual virginity of the blessed mother, no one believed that she had other children until the last two hundred years or so due to widespread mistranslation of Scripture and the non-scriptural practice of Sola Scriptura using improper hermeneutics.
When studying Scriptures as a body of work, in context pointing towards the atonement of humanity there is no other reasonable conclusion that one can come to other than the fact that the blessed mother had no other children than Jesus. Also the historical and the Biblical accounts confirm that the blessed mother was forever virgin. St. Joseph was not chosen as a husband in the normal sense but as the protector of the blessed mother's virginity. He was a much older man, likely a widower, chosen by God for this purpose. Let us look at the verses in question....
(Mat 12:46 DRB) As he was yet speaking to the multitudes, behold his mother and his brethren stood without, seeking to speak to him.
(Mat 12:47 DRB) And one said unto him: Behold thy mother and thy brethren stand without, seeking thee.
(Mat 12:48 DRB) But he answering him that told him, said: Who is my mother, and who are my brethren?
(Mat 12:49 DRB) And stretching forth his hand towards his disciples, he said: Behold my mother and my brethren.
(Mat 12:50 DRB) For whosoever shall do the will of my Father, that is in heaven, he is my brother, and sister, and mother.
The Greek noun used here is adelphoi which means brother and it is spelled exactly the same way both times it occurs in these Scriptures. One should note also that there is no such thing in Koine Greek grammar as a familial form of adelphoi. Now it is important to note that the language used here is in itself a translation from Aramaic for adelphoi used in the familiar way. In Aramaic there is no word which would distinguish ones biological brothers from other relatives such as cousins or even step brothers or sisters. The more acceptable term in proper translation would probably be something like brethren instead of brother. When Christ spoke he also used the Aramaic equivalent to adelphoi in the Greek, in a non-familiar sense when he said the following in verse 50, “he is my brother”.
As you can see with proper exegesis there is nothing in these verses to support the theory that the blessed mother was not forever virgin and that she had other children besides Jesus. Please note that in all of Scriptures, no one is specifically called a child of the blessed mother other than Jesus.
Please note the following:
(Mat 1:20 DRB) But while he thought on these things, behold the Angel of the Lord appeared to him in his sleep, saying: Joseph, son of David, fear not to take unto thee Mary thy wife, for that which is conceived in her, is of the Holy Ghost.
(Mat 1:21 DRB) And she shall bring forth a son: and thou shalt call his name Jesus. For he shall save his people from their sins.
(Mat 1:22 DRB) Now all this was done that it might be fulfilled which the Lord spoke by the prophet, saying:
(Mat 1:23 DRB) Behold a virgin shall be with child, and bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
The blessed mother was a virgin when she conceived and was a virgin after giving birth.
(Luk 1:26 DRB) And in the sixth month, the angel Gabriel was sent from God into a city of Galilee, called Nazareth,
(Luk 1:27 DRB) To a virgin espoused to a man whose name was Joseph, of the house of David: and the virgin's name was Mary.
(Luk 1:28 DRB) And the angel being come in, said unto her: Hail, full of grace, the Lord is with thee: blessed art thou among women.
(Luk 1:29 DRB) Who having heard, was troubled at his saying and thought with herself what manner of salutation this should be.
(Luk 1:30 DRB) And the angel said to her: Fear not, Mary, for thou hast found grace with God.
(Luk 1:31 DRB) Behold thou shalt conceive in thy womb and shalt bring forth a son: and thou shalt call his name Jesus.
(Luk 1:32 DRB) He shall be great and shall be called the Son of the Most High. And the Lord God shall give unto him the throne of David his father: and he shall reign in the house of Jacob for ever.
(Luk 1:33 DRB) And of his kingdom there shall be no end.
(Luk 1:34 DRB) And Mary said to the angel: How shall this be done, because I know not man?
(Luk 1:35 DRB) And the angel answering, said to her: The Holy Ghost shall come upon thee and the power of the Most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God.
The verses preceding show that she intended to remain a virgin when she said to the angel Gabriel, “I know not man”. Please notice in the following verses in St. John’s account of the crucifixion of the blessed mother being put into the care of St. John. If she had other children and especially other male children the Jewish tradition would be that she would have been cared for by them. Jesus knowing the hardship of a childless widow placed her in good hands with St. John.
(Joh 19:26 DRB) When Jesus therefore had seen his mother and the disciple standing whom he loved, he saith to his mother: Woman, behold thy son.
(Joh 19:27 DRB) After that, he saith to the disciple: Behold thy mother. And from that hour, the disciple took her to his own.
Last of all there is the difficulty of history and Sacred Tradition to overcome, which has always, without exception, called her the Blessed Virgin.