Can you use a variation of Anne? Claire Annaleise is really cute. Or what about Annaleise Norah? That's adorable.
Your 1st daughter's name is rather L heavy and with Anne as a middle you probably need something that doesn't end with an A and preferably doesn't have a strong N sound either.
I like Norah. Norah Anne sounds okay, but runs together a bit.
I like Norah from your list, and think Norah Anne sounds fine despite the fact Norah ends with an A sound
I love a previous posters suggestion of Anna Claire. Ellie and Anna or even Ellie and Anna Claire work for me (I live in the South, ha!).
Norah is my favorite from your list with mn Anne.
Love Norah. Or you could do Eleanor with Norah as a nick name
Okay, you like Claire, Lauren, Isla and Norah. Middle name Anne. Sister Ellie. Is Ellie short for something? What is her middle name?
Maybe you will love:
Laurie/Lori Anne - she could go by her full name. I once babysat a girl named Lorianne.
I also knew girls at my high school named Rayanne and Reanne. Could you use Anne in the first name? Breanne, Beth-Anne, Julianne, Leanne, Luann, Ruthanne, Roxanne, Susanne/Suzanne?
Some other names you might like:
Lucille Anne, Lucy Anne, Lulu Anne
That was actually pretty difficult. I didn't want to suggest anything too L heavy and close to Ellie or anything that ended in an A.
I think Claire and Ellie and Isla and Ellie are perfect for siblings!
Claire Anne works a bit better then Isla Anne though, I think, so that gets my vote.