For a variety of obvious reasons, I'm a big advocate of siblings sharing a room at some point in their childhood -- even if space isn't an issue. To that end, Bryan and I have a plan (read: scheme) that involves converting Millie's room into a shared "sleeping space" for the girls in the next year or so and morphing Audrey's into their joint playroom.
I'm already talking up the idea to Audrey to get her sufficiently primed, and so far she's pretty jazzed about the concept. (Although I'm not sure she'll feel the same way a year from now, thus the long prep period.)
But honestly, I think it's the ultimate kid fantasy to have some sort of groovy sleeping space where you can cozy-in together, staying up late telling stories and secrets and, you know, just be sisters...