1. Ivy: My favorite. Short and sweet and feminine. A nice inverted match with Lily so each girl would each have a nature reference. Cora and Ivy just sound perfect together to me. It's spunky and less common and every Ivy that I have ever known has been super sweet.
2. Esme: I love this one too, another beautiful name with 4 letters but completely it's own sound. I can definitely see Esme as Cora's little sister.
3. Ella: I've just always loved this name. It sounds very sparkly to me which is also how I feel about Cora and I think it holds it's own next to her.
I think Ivy is my favorite by a slight margin, but they're all great. Cora is Such a beautiful name, it's always been a favorite of mine. You've had some great suggestions and your short list is looking very nice, I'm sure that any name you choose will sound lovely with big sister's. Good luck! =]