Here are the Bible verses that I am aware of that make reference to Jesus' brothers:
[Matthew 12:46-47 NIV]
46
While Jesus was still talking to the crowd,
his mother and brothers stood outside,
wanting to speak to him.
Does not say Her sons, but His brothers...
She is simply NOT recorded as having birthed ANY other children in Scripture...
47
Someone told him,
"Your mother and brothers are standing outside,
wanting to speak to you."
Same thing... This was early in His ministry, and Hid identity was being kept hidden...
[Galatians 1:19 NIV]
19
I saw none of the other apostles--only James, the Lord's brother.
His was an adoption into another family - Of COURSE they are His brothers...
That is another point... They were the only brothers He ever knew...
All of them older than Him...
Who knows what they knew of Him -
I would guess that, to them, he was simply a late son of Joseph's new wife...
[Jude 1:1 NIV]
1
Jude, a servant of Jesus Christ and a brother of James,
To those who have been called,
who are loved in God the Father and kept for Jesus Christ:
Jude 1:1 - BYZ –
ιουδας ιησου χριστου δουλος αδελφος δε ιακωβου
Jude...
Of Jesus Christ a servant...
But a brother of Iakovos [James]
This is literally what it says [above]...
If Jesus' mother was there, then Jesus had a mother.
Yes...
If Jesus' brothers were there, then Jesus had brothers.
Yes - But biological or by marriage?
I know how my mother gave me brothers ...
Your mother did not give birth to God...
Paul establishes in Galatians that James, the leader of the Church in Jerusalem, is the brother of Jesus.
The Orthodox Church calls him the Brother of the Lord...
But not a blood relation...
Jude, the author of the letter bearing his name, identifies himself as the brother of James
And NOT the brother of Jesus... But a servant instead... There is history for this statement in Holy Tradition...
(the brother of Jesus, head of Church in Jerusalem, and author of the letter of James).
That's him - No DNA kits in those days...
I suspect that if Jesus had "cousins" rather than brothers that there is probably a way to express that in Greek.
The mother of John the Baptist was a cousin of the Theotokos, and she had other cousins...
But the word for cousin is suggeneis - sung-genesis and can mean same family, relatives, of from the same nation...
So the issue becomes one of half brothers and other wives that are never mentioned in scripture (I hate assuming things that are off-stage), plus something of a mockery of the definition of "marriage" if Joseph was "married" to a woman he never, ever slept with.
The marriage was for the sake of Jesus, not vice versa...
And for you sola folks, there is no scripture recording their wedding...
Arsenios
So while I have nothing to PROVE that what the RCC that I once considered joining is wrong about the Perpetual Virginity of Mary (which is a cornerstone of her Assumption), I could never overcome my personal doubts based upon what little information actually appears in scripture about Jesus' brothers.
Thank you for allowing me to explain my beliefs. I hope that I have not offended anyone and if I have misrepresented any details of the Catholic position, it was out of ignorance rather than malice and I welcome correction.