You're assuming also that there were other brothers and sisters and that is part of your argument against perpetual virginity.
Actually, I haven't spoken regarding this point ... today. :^)
If so, why did Jesus defy Jewish law and give over the care of his mother to someone who was NOT his blood brother? Someone who had no legal right to his mother?
Jesus' brothers were not believers at the time of his death.
Would you, as a christian, hand the care of your mother utterly into the hands of non-believers, ... or into the care of a trusted disciple ?
Where do you find Scriptural evidence that Jesus' "brothers" (actually His cousins, but I'll humor you. It's too late at night for a massive refutation of that idea right now) were non-believers at the time of His death? There is none. Cousins or brothers, there is not one shred of evidence that they were unbelievers. According to you guys, at least two of them were Apostles! The Apostles were to have all doubts removed three days later in the upper room, no? So what would have been the big problem?
If He had brothers, Jesus would have defied MASSIVE Jewish tradition against not favoring the eldest son in all things pertaining to inheritance. With Jesus dead, according to your theory, some OTHER man was now the eldest. He would HAVE to care for Mary in this circumstance.
Since this didn't happen, and her care was entrusted to someone we KNOW wasn't His literal brother (John was the son of Zebedee), there is compelling evidence that Jesus had no blood-brothers.
Besides that, we have the testimony of the early Church, which unanimously understood the situation in an identical fashion to modern Catholicism. You would discount testimony from those early days, even when that testimony starts within the lifetimes of those whose lives overlapped the Apostles themselves! Yet, somehow, it is the Catholic Church which is "ignorant" in this matter...