Is that the Merovingian one? It's a bit silly to make it Frankish because the Grail story is associated with the Romano-Britons.
What's more, there is precise genetic evidence for what happened to the Romano-British leadership. For example, look under
https://www.familytreedna.com/public/Randolph%20DNA%20Surname%20Project?iframe=ycolorized for Dennis and Edward Fitz Randolph and Joseph Randall.
Their Y-DNA pedigree derives from ancient Latium and is:
R-P312/S116 > Z40481 > ZZ11 > U152/S28 > Z56 > BY3548 > Z43/S366 > Z144 > BY28794 > PF6582 > PF6577 > FGC36902/BY3953 > FGC36897 > FGC36895 > A8380 > FGC41936
It is consistent with the Sabine origin attributed to the Aurelii by the Roman historian Festus. The Sabine second traditional king of Rome, Numa, is interesting because in legend he introduced much of Roman religion, including the worship of Jupiter and Mars, and the position of Pontifex. The administration of the Roman Church is called the Curia and shares its etymology with the name of the Sabine capital, Cures.
The Aurelii were the priests of the Sun. The Aurelii Cottae were an Equestrian family who served Rome both as Tribunes and as Consuls.
Aurelia Cotta was Caesar's mother and Augustus's ancestor.
Archaeologically, we know of Aurelia Aureliana who lived in Carlisle in the early 3rd century AD, around the time Biblical scenes were being painted in the Hypogeum of the Aurelii in Rome (a catacombs-like burial space for this gens).
Aurelia Aureliana's widower was Ulpius Apolinaris. Two centuries later, the Bishop and Senator Sidonius Apollinaris was a friend of the British leader with the title Riothamus, who has been identified with Ambrosius Aurelianus.
Aurelius Ambrosius is also the name of St Ambrose, who advise the usurper Magnus Maximus not to execute Priscillian. (Note: Aurelii weren't particular about name order, as the inscriptions in the Hypogeum show.)
The reputed descendants of the Aurelii were the House of Vannes/Rennes, dukes of Brittany. They founded Parliament and advised William the Conqueror to ameliorate his cruelties to the English. Their descendants were counsellors to English (e.g. the FitzHugh family) and to French monarchs for centuries (e.g. Cardinal Richelieu claimed a descent from them).
The FitzRandolphs and Randalls claim to be male-line descendants of those dukes, and many US Presidents have FitzRandolphs as recent ancestors.
Plenty of real-life substance to chew on there, without bringing Clovis into it.
