A lot of debate occurred on the time on if it's fictional or factual. Even ICAR had to step up and claim it's all fictional.

One fact we have is that on south of France it's been tradition for 2 thousand years that Mary Madalene hid there after the crucifixion.

All the myths of the sang real / blood line are old and not created by Dan Brown. There's even an older movie that also depicts the heir discovering who he is after his father is killed by ICAR minions.

What Dan Brown created is the story characters and modern events that happen on the story. He basically creates it by the thought "if the myth is real and centuries ago the heirs were hunted and the Grail was recovered and hidden, where would it be now and where would the heir and his guardians be?".

Of course, on ICAR's books there's no evidence that Jesus was married or had any child. Lots and lots of modern-relevant matters are left out of bible, in example ecology and homosexuality. There are passages talking about his father and mother and brothers and primos. On agnostic books there's more info about Mary Madalene, but still nothing claiming they were married. They could pretty much had been lovers and never married. But most probably she had just been a close disciple.

It would had required a lot of work from all apostles and disciples to hide the marriage and children if they were real. As we know, after Jesus death, they disbanded and each disciple moved to a different place and they pretty much never met again. They'd have all to agree on hiding it and never talk about it to anybody for decades, without Peter being able to talk to each of them to reassure the silence.

The marriage and heir would had been of supreme importance matter, specially a few decades later when they'd figure that Jesus returning would take longer than expected, that they were gonna die and a heritage of new generation would be needed to keep the evangelism work.

At some point, some apostles and other disciples would make word on the existence of the blood line, which would eventually end on some writing, maybe even on books specially on the matter.