(Sorry in advance for any perceived dehumanization of your grandma)
This is not only a vote for deterministic tendencies, but also a function of architecture.
Assuming she only expressed the non-glitchy neural pathways available to her (since if she accessed a glitchy pathway it would fizzle and not rise to consciousness/motor cortex speech expression), she already has a smaller subset of tree paths available to her to begin with.
Depending on the dementia, she could theoretically have fewer and fewer functional pathways available over time. That makes the sa...
Having learned about this game right now, this is my idea to set up a random deck (so not field-tested, and maybe I'm misunderstanding):
Separate into 4 piles of 12 cards, according to suit, face down.
Another person moves the piles about like in 3-card Monty until no one knows where each suit is.
Randomly choose 3 piles to remove 2, 2, and 4 cards. Or always remove 4 from the first, 2 each from the next ones, from left to right or by whatever convention.
Shuffle together remaining piles, deal.