I believe that Paul referenced it in his epistles and many of those where written long before Mary died. Some of the earliest copies of Paul's writing date to about 20 to 30 years after the death of Christ. Mary was probably about 48 to 50 when Christ was crucified. Also it is believed that James, the brother of Jesus, was a older half sibling who was alive for the birth, death and ressurection of Jesus. He was martyred for his teaching of Christ's birth, death and resurrection. How likely do you think it is that he would have gone to his death lying about something that he knew the truth of?