I've been doing some digging through episodes related to the Casterwill family. One thing I noticed, and that I think many may have, is that the houses of the Casterwill family seem to correspond with the original children of Casterwill. Well, almost.
Several connections are relatively obvious, such as tracing back the House of Forgers of Weapons through the Ladies of the Lake to Lord Casterwill's unnamed elder daughter. Others aren't so obvious
But - the original Casterwill family comprised Lord Casterwill, his Champion, and four unnamed children. The current Casterwill family elders comprise Sophie with her Champion and five houses.
My speculation, based on how close Lord Casterwill was to the Betrayer before, well, the betrayal, I wonder if this were actually due to a schism in the family. My suspicions are furthered by the existence of the perfect House, the House of Curses and Secrets, formally known as the Rune Guardians. I must note Lok's unease about this house, and even Sophie seemed to be trying to convince herself when she said that the Rune Guardian must have good intentions.
My attempt to recreate the family history based on the houses yields the following - though I also found nothing to support the elder/younger classifications that currently exist for Casterwill's kiddos. Note that this is speculative and should not appear on articles without episode sources. Lancelot goes… somewhere.
- House of Nobles
The House of Nobles are the direct bloodline of Lord Casterwill. Presumably, this house goes through his elder son (I'd presume the wiki got that wrong and that it should be the one paired with Mythras). This house pretty much has to be represented by the lion Titan, Mythras. Through Lord Casterwill, the house is also represented by the Amulet of Will. The house's lineage includes both Lucas and Sophie and at least one of their parents.
- Champion of Casterwill
Not much is known about the Champion apart from the fact that the current Champion, Lok Lambert, isn't a part of the Casterwill family. The Legendary Titan representing the Champion is Pendragon. As King Arthur wielded a sword bestowed by a Lady of the Lake, he probably was a historic Champion.
- House of Blade Makers
The House of Blade Makers, being led by the Lady of the Lake, most likely goes through Casterwill's "elder" daughter. This house was tasked with keeping the Willblade, seemingly to bestow it upon a Champion. The Legendary Titan representing this house is Quetzalcoatl. In Medieval times, this included the contemporary Lady of the Lake. This line includes Nimue and Viviane and probably also includes Mallory and White.
- House of Strategists
The House of Strategists is difficult to place. For gendered parallelism, I'll pick the "younger" daughter. Phoenix isn't a bad fit for this house, now that I think of it. Teien is the modern representative of this house.
- House of Knowledge
The House of Knowledge is also difficult to place, so I'll parallel by using the remaining son. The short-haired one with Umbra's ring seems to better fit the house as well for some reason. Though described as a house of knowledge, they seem a bit more like historians. Joan of Arc, with Arc, would make good members of this house. The lineage most likely included Nostradamus, the Seer, who bears an uncanny resemblance to Focauld, the current elder.
- House of Curses & Secrets
The House of Curses may have originated with the Betrayer, himself. What better house to deal with the dark secrets of the Nullifiers and then betray the rest of the family? Interestingly enough, the colors of the Rune Guardian's (Rune Guardians') robes is nearly a match to those of Void. Having Blood Spiral magic originate from this house would also explain why many are so powerful and less publicly-known. No Legendary Titans are known to be associated with this house, though the Betrayer made use of Demigorgan and Legion.