Mary and the others were said to have brought the grail with them — a symbolic way of saying that Jesus' child had come with them, Starbird said.
In Leonardo's painting, the figure is wearing the same color as Jesus — possibly indicating a special relationship, Brown said. He said the way the figure is aligned with Jesus also forms a V, a symbol of the feminine long before Leonardo's time.
Many art historians have dismissed the theory that the figure is a woman, saying it's just a tradition to paint John as beardless and long-haired. "It looks like a young male. I see no breasts," art historian Jack Wasserman told ABCNEWS.
But Carlos Pedretti, one of the world's leading Leonardo experts and head of the Leonardo Institute in Florence offered a rare word of agreement, noting that a portrait of the figure next to Jesus, sketched by one of Leonardo's top students, clearly appears to be a woman.
Stories about the Priory of Sion first surfaced in the modern era during the late 1960s, when a set of documents was discovered deep in the French National Library that made numerous references to the supposed society.
The documents outlined a family tree that went back to the Merovingian Kings, monarchs who ruled in the south of France from the 6th to the 8th century.
According to Henry Lincoln, who co-authored the 1983 book, Holy Blood, Holy Grail with Michael Baigent and Richard Leigh, the Merovingian legends say the first king's mother was said to have been impregnated by a sea creature. One of the earliest symbols for Jesus and Christianity was a fish.
Holy Blood, Holy Grail suggests that the Merovingian kings were the descendants of Jesus and Mary Magdalene, and that their descendants founded the Priory of Sion. The book argued that the Knights Templar, an order that really existed in the 12th century, originated as the military wing of the Priory of Sion.
In the earliest written versions of the legend of the knights of the Holy Grail, the knights were called Templaisers, which historians think actually refer to the Knights Templar.
"They were referred to as the Guardians of the Grail and of the Grail Family. They were there to protect that royal family," said Lincoln.
The Knights Templar existed for 200 years, but were suddenly rounded up one day by King Phillip of France, accused of heresy and executed by the priests of the Inquisition.
One theory is that the Knights Templar were destroyed because during the crusades, they might have brought back evidence from the Middle East that proved the identity of Mary Magdalene and her descendants, Brown said.
Although Brown based his novel around the legend of the Priory of Sion, he is not sure himself whether the mysterious society really existed or is a modern fabrication. "I don't think we'll ever know," he said.
Using the Priory of Sion documents from the French library, ABCNEWS found a pair of noble-born Scottish cousins named Andrew and Niven Sinclair.
Their ancestor William Sinclair built the Roslyn Chapel in Scotland — often referred to in the legends of King Arthur as the Chapel of the Grail.
As far as their holy roots, Andrew doesn't put much stock in the legend. "There's no proof that actually this is a bloodline of Mary Magdalene and Jesus," he said.
Niven Sinclair does think they married into the Jesus bloodline. But, he said, "by the time it gets to me, it will be very, very, very diluted."