If you both agree on Isabelle, why not go for it? It's a lovely name, goes well with Eva and has a variety of possible diminutives.
Other ideas:
I know Evas with sisters called:
Anna, Isla, Zia, Annalise and Isabella
I also think it goes well with:
Abigail, Charlotte, Clara, Darcey, Flora, Felicity, Grace, Holly, Hope, Imogen, Jasmine, Katrina, Katie, Lydia, Matilda, Miranda, Nina, Olivia, Phoebe, Ruby, Rosie, Rebecca, Sophia, Sophie, Scarlett, Tilly, Verity, Willow