I'm re-reading Malory and it strikes me that it would be incredibly useful to have as a reference during play how all the main characters are related. I understand there is not single source and that there are contradictions, but I would hope a general consensus could be arrived at that would provide guidance on the most important knights.

Does such a thing already exist?

No consensus as far as I know.

Try to get hold of The King Arthur Companion by Phyllis Ann Karr (1983), it contains a lot of information about major and minor characters, and some nice family trees for the major "clans".

Actually, keep your eyes peeled for The Arthurian Companion, Second Edition from Green Knight Publishing. It was a revision and expansion of The King Arthur Companion, to such a degree that the author felt it proper to change the name.

It is a great book to have to hand.
Perhaps we should also gradually build up the resource ourselves using the Pendragon wiki: http://www.gspendragon.com/roundtable/index.php?topic=1232.msg9883#msg9883

Actually, keep your eyes peeled for The Arthurian Companion, Second Edition from Green Knight Publishing. It was a revision and expansion of The King Arthur Companion, to such a degree that the author felt it proper to change the name.
If you have the chance to find this via Amazon.com or Half-Price Books or where-ever, BUY IT. It is a fantastic book and a great resource for information Arthurian, even if you are not playing Pendragon.