Before I present a few family histories I should explain a couple things. Living in an age where a person was judged by his father’s accomplishments and often addressed by his patronymic, Arthur is not consistently known as the son of Uthr/Uther in British literature. Peredur/Perceval is originally known as the son of Efrawg, a Cymricization of Latin Ebrauc, modern York; it should come as no surprise that Perceval’s father changes with the author. Lancelot is consistently the son of Ban/Pant, but he was either invented by Geoffrey as a nod to events current in his time or (and more likely) by Chretien. In the latter case his name originally was L’Ancelot, the servant, as it appears on several manuscripts from the oldest originals. Modred is either the son or the nephew of Arthur, though originally he appears to have been an independent king who was either an enemy or, more likely, an ally at Camlann.
That said, there are three major families in the Arthurian universe, Arthur’s, Perceval’s, and Lancelot’s. Arthur first. A -, /, or \ indicates the next generation, thus Amlawdd had three children:
Pre-Galfridic Welsh sources
Amlawdd-Uthr- Arthur- Amr
Note that Gwalchmai is not here. He was made Arthur’s nephew with Geoffrey of Monmouth, and had no siblings at that time. Chrétien de Troyes and Wolfram von Eschenbach made no further familial connections, but Gawain begins to have brothers with those who followed. Women were considered of no personal significance (apart from their symbolic connection to the land), and so it is common for there to be multiple names for a hero’s mother. Instead of listing them I’ve given the men his sisters married, who are consistent. Arthur’s family continued to develop until the Vulgate version of the Arthurian Romances, where it crystallized:
Urien was a king of Cumbria in the middle or late sixth century. Loth ruled Lothian about a generation earlier. Arthur, as I’ve said before, ruled in the late fifth or early sixth century. Next, Lancelot’s family:
Bors-Elyan the White
Lanzelet makes Lancelot a maternal nephew of Arthur, but that seems like an author-specific connection as it disagrees with every source before and after. Perceval’s family is last:
You will recall that the name of Perceval’s father was never known. The grail was actually a device used by nature worshippers (see my ‘Origins of Arthurian Romances’), who followed the fertility god Belatacudros, euhemerized into the legendary Welsh king Beli and bastardized on the continent into Pelles and Pellinore, among other characters. And that is why Perceval, associated with the grail, eventually came to be associated with Pellinore.