I actually think Atlas and Sailor make a really fun sibset! They make me think about sailing around the world, since an atlas is a book of maps. In fact, most people will probably think of atlases first when they hear the name, rather than the mythological character, so word names like Juniper, Marigold, and Vesper (love!) seem like a good fit to me.
But a mythological theme would also be lovely. Penelope immediately came to mind. Penelope and Atlas - they just go together somehow! But maybe Penelope is too long and/or too popular for your tastes. A pp mentioned thinking Juno overpowers Atlas, but I think they go rather well together. (You could even do Juniper, nn Juno.) How about Thora, Tanith, Venus, Danae, Leto, Helen, Irene, or Luna?
That all being said, most of the names on your list seem fine to me as sisters to Atlas. It's totally okay for sons and daughters to have different styles of names. The only one I would suggest scratching is Jane, which seems really plain next to Atlas, but it would still make a nice middle. Oh, and Iris. Although it would fit with a mythological theme, I think the endings sound too similar.
Thank you all infinitely -- these responses were incredibly helpful. I particularly liked:
Elektra was the name that really stood out to me on this list. Gorgeous, quirky, mythological -- perfection!
@mega_muffin: You made me fall in love with Atlas and Sailor as a sibset! You're right -- they both have an air of adventure and travel. I agree that Jane seems too, well, plain, and while I love Atlas and Iris, I also worry that they're too similar.
Fantastic suggestions! Thanks again. :)