Do you really want to make the argument that the number of believers or the amount of money held by the religious organization determines the validity of the belief system?
Many people in the past have been able to convince others that he/she is the way to heaven, and to become rich in the process. Recent ones that I can think of include Joseph Smith, Jerry Farwell, Pat Robertson and José Luis de Jesús Miranda. The last one states to be both the Antichrist and Christ. All have received significant media attention will little or no evidence that their beliefs systems are correct. All have also conned thousands or millions from their money. All are believed to be false by you.
One thing that is interesting to me about the above names is that all preach that they must be believed on faith. They all claim to have seen what they preach due to direct conversations with angles or God himself, something that can never be proved or disproved. However, claiming to be the descendent of Jesus and Mary Magdalene is something that can be tested and tests will show that it is impossible to give evidence for the claim. In this case, either the media or independent journalists will attempt to test it.
Perhaps we should try though and put to rest this issue? Go out and proclaim to be the descendent of Jesus and Mary Magdalene to see what will happen. Then go out to try to start a new religion. I bet starting a new religion will be much easier and conmen generally try to take the easiest approach ;).
By the way, a simple google search (first hit) will lead you to a claimed descendent of Jesus and Mary Magdalene here, while another 2 seconds of searching will lead you to a cool test to help you determine if you are a descendent of Jesus and Mary Magdalene. Sadly, I am not :(. Well, actually I got bored with the test and stopped. I assume that means I am not a descendent.