[name]Hi[/name] there! 
If it’s okay with you, it would really help if you could tell us your daughter’s name, so we could make sure it will work with the name of her new sister.
[name]Leela[/name]/[name]Lela[/name] (pronounced lee-lah): I actually know a [name]Lela[/name] (spelled [name]Lela[/name]), who pronounces her name “lee-la,” and has spent a good chunk of her life correcting people who call her “leh-la.” What about [name]Lila[/name]?
[name]Evangeline[/name]: I really, really love [name]Evangeline[/name]!
[name]Thea[/name]: I think [name]Thea[/name] is pretty, but you’ll encounter pronunciation issues (thay-uh, thee-uh, tay-uh). I think it’s a great nickname for [name]Theodora[/name] and [name]Dorothea[/name].
[name]Luna[/name]: Like [name]Erin[/name], I’d be concerned about looney nicknames and teasing.
[name]Anais[/name]: I’m going to second my dislike of [name]Anais[/name] based on it being too close to that, um, part of the human anatomy. I know how to pronounce it properly (ahna-eese), but anyone who isn’t familiar with the French pronunciation would likey pronounce it ann-ace.
[name]Scarlett[/name]: I really like [name]Scarlett[/name], and think [name]Lettie[/name] is a cute nickname. That said, I’d personally keep it as a middle due to the potential “scar” nickname.
[name]Freya[/name]: I’m not a fan of [name]Freya[/name] (I just don’t like the sound, and I think of frayed clothing), but that’s just me!
[name]Annabel[/name]: [name]Annabel[/name]/[name]Annabelle[/name] is one of my favorite names, but it’s getting really popular.
[name]Amber[/name]: [name]Amber[/name] is really, really dated/out of style. 
What about [name]Pearl[/name] or [name]Ruby[/name]?
[name]Charlotte[/name]: I really love [name]Charlotte[/name], but it’s really getting popular (it’s #87 now), so it’s no longer unusual.
[name]Mabel[/name]: I think [name]Mabel[/name] is darling!
[name]Kitty[/name]: I think [name]Kitty[/name] is a great nickname for [name]Katherine[/name], but I think it’s too unsubstantial as a name in and of itself.
[name]Tilly[/name]: Another great nickname for me, but not as a stand-alone name. I really like it as a nickname for Matlida.
Other ideas:
[name]Aurelia[/name]
[name]Beatrix[/name]
[name]Clementine[/name]
[name]Celia[/name]
[name]Cora[/name]
[name]Cordelia[/name]
[name]Clover[/name]
[name]Daphne[/name]
[name]Dahlia[/name]
[name]Delphine[/name]
[name]Eliza[/name]
[name]Flora[/name]
[name]Felicity[/name]
[name]Georgiana[/name]
[name]Genevieve[/name]
[name]Helena[/name] (nn [name]Nell[/name] or [name]Lena[/name])
[name]Hazel[/name]
[name]Ivy[/name]
[name]Isla[/name]
[name]Juliet[/name]
[name]Lila[/name]
[name]Lydia[/name]
[name]Louisa[/name]
[name]Margaret[/name] (nn [name]Daisy[/name], [name]Maisie[/name])
[name]Phoebe[/name]
[name]Paloma[/name]
[name]Romilly[/name]
[name]Simone[/name]
[name]Tamsin[/name]
[name]Willa[/name]
[name]Zara[/name]
Good luck!
Respectively they are at 450 and 623 on the social security list. While they are definitley more popular than they used to be, its not overly so. I think you’ll be able to go many places without running into another little [name]Evangeline[/name] or [name]Isla[/name]. I think the only time it really becomes a problem is when the popularity is under 100. Then you run the risk of having a few in your child’s class. That’s the thing that bothers me the most. And while both these names are increasing in popularity, I don’t see them soaring to under 100 any time soon. I say, these two names are on your short list for a reason. Go for it!