I would shy away from Nolan & Ryan together, myself. I really liked the suggestions Ellery or Linden. Others you could try out:
Lindley (or Lynley/Linley - all are pronounced LYN-lee), Dana, Aaren, Arden, Afton, Brett or Britt, Brynn, Carey, Devyn (or Devin/Devon), Garnet (not very unisex but also not commonly used and "stronger"-sounding gemstone name), Greer, Hollis (can go by Holly), Morgan (popular, but still nice), Regan (pron. REE-gan, esp. nice if you have an Irish background), and Sage.
Also, had one family not moved away while my female Ryan was in Jr. High, there would have been four female Ryans in the school at the same time.
I think since neither can agree on a name they should go with their orriginal name of Charlie and I love the name Charlie for a girl. My daughter is named Ryan and she is 35 years old and she's had no problems with it. It has not hampered her in the work field at all. When she was watressing shewould run into old couples that would ask her about her name. She would tell them my parents wanted a boy. The older couples would always say, "oh, Dearie, I'm sure they're glad they had a girl". She would tell them she loves having her name. It's unique and it just fits her.
wacky because while there's unisex usage of Ryan, think it's too rare. too many people don't even have Ryan on their girl radar at all. in a girl unisex sibset maybe, but not next to a brother.