LIke all the characters in the saga, the Cat and the Kitten have been through a vetting process in "Character Development". The discussion on the Kitten (which includes the issue of Personal Vis Source) starts here:
https://forum.atlas-games.com/t/character-development/5173/565
Quickly, though:
The cats don't belong to Viola--they came with her from Andorra. They're well-disposed to her, but I don't expect them to accompany her around.
The cats are Companions--my Companions. I did, however, let someone else play the Cat when she accompanied Viola in exploring the temple regio, so that I wouldn't be playing both characters.
The Cat and the Kitten are two, separate, and quite different characters. There's a reason there are two of them-: they were meant to play different roles in the story I had in mind when I created them for the Andorra saga, before things went bad there. Because of their different personalities and goals, I expect to play them separately in the future. They appeared in the barrow story together, because both had reasons to be there, and both had useful capabilities, but the SG in that case really didn't know what to do with them. You'll note that only the Cat came along on the temple story. I've got a new story in mind that will feature the Kitten, but I'll need to discuss it with CH or Salutor.
Personal Vis Source is normally a Hermetic Virtue. I think, however, that's not because of some mystical restriction (most other Hermetic Virtues are clearly Hermetic because they affect the use of Heremtic magic), but because, in the original rules, magi were the only characters who could make use of vis--I'm reasonably sure the Virtue would be suitable for a hedge mage Companion, for example. In the case of the cats, the sole use of the vis is to allow them to earn XP's. It's not Viola's, it's not the covenant's, and any magi wishing to use it would likely have to pry it out of their cold, dead hands.
Scott