I contacted Robin, and happily she wrote back.
This was her response:
"I don't think of the cards in the Major Arcana as being associated with any suit. They are, for all intents and purposes, their own suit. Except that, really, each one stands alone, as the crystallization of a concept. An archetype, if you will.
In my tradition, anyway, the four suits are associated with the four elements. (Earth, Air, Fire, and Water.) If the Majors are associated with anything, they are all associated with Spirit.
But, although I drew the deck, and put a fair amount of my thoughts and so on into it, the one you are holding in your hands is *your* deck.
What's really important are the associations that *you* attribute to each of the cards."
This really adds another dimension for my own study!