The Perpetual Virginity of the Blessed Virgin Mary.

According to the Word of God...

If Jesus had been the eldest among children then the others would have been responsible for taking care of their mother. This was not true of step children, who would not have had that responsibility.

If, as some ancient sources say, Joseph was a much older man and a widower with children by his previous wife, then this passage makes perfect sense.

Why then was one of the last things that Our Lord did from the cross the act of assigning his mother to the care of a non-family member?

If He had blood siblings instead of step siblings (who were not responsible for their step mother), why would he have done that?

John 19:26 When Jesus therefore had seen his mother and the disciple standing whom he loved, he saith to his mother: Woman, behold thy son. 27 After that, he saith to the disciple: Behold thy mother. And from that hour, the disciple took her to his own.

Furthermore, it was scandalous in Jewish society for a younger sibling to rebuke or mock the eldest, yet this is what we see happened in the New Testament. However, if His step brethren were older than He then this might occur. Look at Jacob's sons and Jesse's sons in dealing with Joseph and David.

Also, it was virtually criminal for a younger sibling to mock or rebuke the first born son, and yet the word of God shows that this happened to Our Lord This would only be possible if they were the older step siblings by another mother.

With historical documents that suggest that Joseph was a much older man and a widower with children, your assertions fails.
'Brethren of the Lord'
Mary: Ever Virgin (Fathers)
If Mary had other children, then where were they the day their eldest brother Christ was Crucified? Their mother was there and Jesus' best friend John? If she had other kids they would've been there too and they would've been responsible for taking care of Mary. Instead, from the cross and with almost his last breath Jesus gives her to St. John. Something that he wouldn't have done under Jewish customs back then IF HE HAD SIBLINGS.
The New Testament clearly indicates that they were his cousins...the children of Mary's cousin Mary (John 19:25-27).
"25 Now there stood by the cross of Jesus, his mother, and his mother's sister, Mary of Cleophas, and Mary Magdalen.

See! Even his mom's cousin was there...where are these supposed siblings of his? Nowhere...that's where, because they simply did not exist.

Even all three of the "pillars of the reformation" wrote in support of it.
My thanks to San Juan Catholic Seminars for publishing this in their Beginning Apologetics # 5 booklet.

The Perpetual Virginity of Mary:

Martin Luther: “ It is an article of faith that Mary is Mother of the Lord and still a virgin… Christ, we believe, came forth from a womb left perfectly intact.” ( Works of Luther, Vol. 11, pages 319-320; Vol. 6, page 510.)

John Calvin: “ There have been certain folk who have wished to suggest from this passage [Matthew 1:25] that the Virgin Mary had other children than the Son of God, and that Joseph had then dwelt with her later; but what folly this is! For the gospel writer did not wish to record what happened afterwards; he simply wished to make clear Joseph’s obedience and to show that Joseph had been well and truly assured that it was God who had sent His angel to Mary. He had therefore never dwelt with her nor had he shared her company… And beside this Our Lord Jesus Christ is called the first-born. This is not because there was a second or a third, but because the gospel writer is paying regard to the precedence. Scripture speaks thus of naming the first-born whether or no there was any question of the second.” (Sermon on Matthew 1:22-25. Published in 1562.)

Ulrich Zwingli: “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.” ( Zwingli Opera, Vol. 1, page 424.)

More info:

Other ECF quotes about the Blessed Virgin:

Other info about Catholic teaching about Mary