When I was a Harvard Freshman in 58-59, I took the required freshman English class and the instructor was an expert on Gatsby....Now, how can my correspondent believe that I randomly picked a sentence with 2 women in white? But, on my purest honor, I did. We're focusing on sentences, so I don't know or care whether Daisy was one of the 2 women. I won't presume, though I will presume that the 2 entities known as "They" are women, given that they are wearing dresses. We must bring our knowledge of what is possible and what is probable to the enterprise of reading, even as we bear down on an isolated sentence. One or both of "them" might be a transvestite male (or a nonhuman), but I'm going to presume 2 women (or girls).
At one point while we were reading Gatsby for the class, he remarked "Have you noticed that whenever you see Daisy in the novel, she is wearing white?"
The "they" is perplexing in another, more disturbing way, because it reappears halfway through in "as if they had just been blown back." We're given a simile that asks us to picture the women, in their white dresses, flying around the house at some earlier moment. They — the women — look like they just landed, as their dresses are "rippling and fluttering" from a recent "short flight." But to say "their dresses were rippling and fluttering as if they had just been blown back" is to create ambiguity, the possibility that the "they" was "their dresses," and we might feel called upon to picture the dresses, by themselves, flying around the house before getting blown back onto the 2 erstwhile naked women. The flying-around-the-house image is fantastical, so we can't tap our our knowledge of what is possible and what is probable, and yet, somehow we know it was the women in their dresses who seem as if they'd just flown around the house and gotten blown back in.
I think the problem of 2 possible antecedents for the second "they" is a writing error, and this Gatsby project is premised on the greatness of the sentences. I hate to be the one to have to say a good editing eye would have seen that ambiguity, but the greatness of the sentence-writing doesn't require a complete absence of error, and the logic of the sentence precludes the dresses flying around the house on their own because we can't picture the dresses getting back on the women without losing the "rippling and fluttering" action caused by the flight and landing. So enough of that. Stop picturing naked women waiting while their dresses fly around the house.
It was the women, so magical and light, like birds or butterflies, that flew around the house. They could fly, but they didn't fly far, only around the house which they got blown back into. These women don't have much ambition or power on their own. They are housebound, even though they can fly. They do an orbit of the house and then a breeze sweeps them back in. But here they are, so pretty in their fluttery white dresses. And of course, they only look as if they'd taken that charmingly domestic flight. The truth is they are sitting together in the house, and they haven't been going anywhere. But there is a breeze, a breeze that might blow a butterfly into the house, and it ripples their flimsy dresses.