I've been following this, and can't wait to hear more from the OP about what her sis decided to do- But, in the meantime, I thought I'd give my experience in Store Bought Turkey Dinners.
I worked at Kroger's, and in order to get Thanksgiving day off, I would work the night shift the Tuesday and Wednesday before Thanksgiving packing dinners. We had them sorted by what was in it, not by name. So we'd have orders for 200 standard dinners on Wednesday, and we'd pack up 250 or so and keep them in the cooler. Then we'd do the specials (ones with odd sides or whatever), these would be labeled by name. And repeat the next night... We generally had people picking up dinners Wednesday, Thursday and Friday, and a smaller number of orders for Saturday.
Based on that experience, had I been the OP's sister, I would have thought that calling on Friday, I'd have a good shot at getting a dinner, especially since there should be one that hasn't been picked up.
We frequently had customers come to the counter wanting a dinner without preordering, and that's why we always had a few extras on hand. And whatever dinners didn't get picked up, went into the hot bar as turkey pieces, or casserole.