Translators are trained to be aware of the ambiguity of certain phrases when attempting to translate from language to another...they have to study contextual usage...especially when it comes to masculine and feminine tenses. Mark 6 specifically mentions brother and sister. Are your prepared to argue that the translaors 0f some 400 years ago were completely incompetent?
There have been some modern tranlations where they went back over all the available Greek, aramaic, and Latin texts;armed with more scientific methods of translation, they still translate those passages as BROTHER and SISTER.
In Luke...Elizabeth is described as Mary's "kinswoman" or what we would describe as "cousin".(as written in the King James...other tranlations have it as "relative"). Now why did the early tranlators(translating originally fron Aramaic to Greek granted) make the distinction between Elizabeth as "kin" but not the aforementioned Brothers and sisters Christ as Kin,hmmmm? Perhaps because when they studied the methods of translation, they studied common contextual usages of the language as well...! Or are you accusing the early tranlators of the Aramaic to the Greek of blasphemy as well...these early tranlators who would have known how and when to call kin, kin and brothers and sisters as brthers and sisters in the familial sense?
The Greek word adelphos is used in the NT to refer to both blood brothers (Mt 4:21-22) as well as kinsmen/associates (Acts 9:17). So we are going to have to look at the context to find whether the writer means "blood brother" or kinsmen.
In Mark 3:31-35, Jesus used the term "brothers and sisters" in a broad spiritual sense - "Jesus said to them in reply, 'who are my mother and my brothers?' and looking around at those seated in the circle, he said, 'Here are my mother and my brothers. For whoever does the will of God is my brother (Gr. adelphos) and sister and mother."
A number of times in the King James Bible, people are called each other's "brother" when they are merely "kinsmen". "And they took Lot, Abram's brother's son, who dwelt in Sodom...And he brought back all the goods, and also brougth again his BROTHER Lot." Gen 14:12, 16). Here is obvious proof, since the term, BROTHER is defined within the context. Thus, adelphos can mean either brother or kinsmen.
In Luke 2:41-52, the story of the 12 year old Jesus lost in the Temple, there is no sense of another child there.
The biggest Scriptural argument against Mary having other children is when Jesus, from the Cross, gives His mother to the care of John. This action makes no sense at all if Jesus had blood brothers and sisters (especially considering the closeness that Jesus and Mary had for each other - as she followed Him around during His ministry).
And finally, the writings of the Church Fathers never mention a blood brother of Christ. The first person to even mention the possibility is thoroughly refuted by Jerome before 400 AD. St. Basil, about 350, said "the friends of Christ do not tolerate hearing that the Mother of God ever ceased to be a virgin". Even Luther and Calvin believed that Mary was a perpetual virgin.
It is a shame that you feel the need to doubt the virtue of the mother of our Lord and Savior. I wonder how Jesus feels about this?
Regards