Elizabeth,
It says; "before they came together".
If someone says to you "Jane Public had no children before she died.", then are we to presuppose that she had children after she died? Of course not.
So then you can see that saying Mary was with child "before they (Mary and Joseph) came together" is only emphasizing that Joseph was not the biological father of this child. It verifies nothing in regards to the argument that Jesus had siblings.
You're making a mistake common to the English language, especially common around here.
Well, how about this...
Mat 1:24 Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife:
Mat 1:25 And
knew her not till she had brought forth her firstborn son: and he called his name JESUS.
The word "knew" here is...
γινώσκω - ginōskō
And Thayer has this...
G1097
γινώσκω
ginōskō
Thayer Definition:
1) to learn to know, come to know, get a knowledge of perceive, feel
1a) to become known
2) to know, understand, perceive, have knowledge of
2a) to understand
2b) to know
3) Jewish idiom for sexual intercourse between a man and a woman
4) to become acquainted with, to know
Part of Speech: verb
A Related Word by Thayer’s/Strong’s Number: a prolonged form of a primary verb
Citing in TDNT: 1:689, 119
The other definitions don't fit here, Joseph knew who she was prior to the birth of Jesus, so to say it had to do with becoming acquainted with here is fallacy.
The word "till" here means...
G2193
ἕως
heōs
heh'-oce
Of uncertain affinity; a conjugation, preposition and adverb of continuance, until (of time and place): - even (until, unto), (as) far (as), how long, (un-) til (-l), (hither-, un-, up) to, while (-s).
And means that the act was consummated after a certain time had passed or event had taken place. In this case the birth of Christ was the event and Joseph did not "know" Mary UNTIL that even occurred. So what happened after the birth of Christ?
Mat 13:55 Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas?
Mat 13:56 And his sisters, are they not all with us? Whence then hath this man all these things?
Joseph and Mary did exactly what married people do and had the results to prove it.