When they knew

Rose

Rose gets in on the plan for obvious reasons, being that she would get her life back and she wouldn't go to jail. It ends up being pretty fun. Debbie's obviously thought the plan through very meticulously and Rose is actually confident that it's going to work out without a hitch. Dressing Daphne Kluger for the Met Gala isn't shabby either.
She's the first one they bring in, and for a while, she's the only one. It's a little lonely, because Debbie and Lou stick to each other like glue and it's evident from the get-go that they have years upon years of history between them - the kind of thing that an outsider could never even touch. They argue and they yell and they disagree hotly on a lot of things, but Rose watches them sometimes over her mannequin and just observes. There's so much trust strung out between them, so much loyalty and devotion. They never kiss, they never say I love you, they never do anything typical to 'normal couple', but a blind man could see it. And Rose definitely does.  So it's never really a matter of figuring it out. It's right there and she sees it and she accepts it, and that's that.
Only it isn't, because after years and years of not wanting anything other than sponsors and the perfect design and two houseboats on the Seine, she wants that. Not either of them, but what they have. She thinks she might want somebody to look at her like she's home. Might want to look at somebody the same way. They fall into each other like it's the easiest thing in the world, and Rose wants something to come easy. For just once in her life.
(Three weeks before the Gala Daphne breaks down crying in front of her and Rose feels her heart swell and sink and ache with the sheer inevitability of it. She designs Daphne's dress and stays up late into the night sewing furiously, and when the Toussaint arrives, she takes a step back to just admire the sheer beauty of Daphne in that dress, in that necklace. Leaving her without having kissed her once is the hardest thing in the world - and then she comes back, storms right into their headquarters like she belongs there, and suddenly, it's as easy as breathing.)