At present, I see no reason to believe or disbelieve its authenticity.
I am Catholic, and I agree with you on this point. As a Catholic, however, I believe:
1) Mary was ever-Virgin and had no children after Jesus. This is supported through the earliest testaments of church fathers from the 1st and 2nd centuries.
2) Any references to "brother" in the Bible can also be interpreted as "cousin", "bretheren", or even believers in Christ as the Messiah (for example, all Christians are brothers and sisters in Christ).
3) Joseph may have been married previously, as many theological scholars believe that he was must older than Mary. Some "brothers" may be Jesus' half-brothers through Joseph.
But the point is, whether the "brothers" were "cousins" or half-brothers is irrelevent. Faith demands that we believe that Mary was ever-virgin; who these "brothers" were is irrevelant to our salvation.
God bless.