Isla & Alice is fine, Isla & Heidi too similar.
It doesn't have quite the classic feel of Alice but you could always go with Isla & Alison to differentiate further. Probably fewer baby Alisons around than Alices!
If it helps, female siblings of Islas announced in the Telegraph in recent years include: Poppy, Talulah, Mia, Katie, Emily, Phoebe, Florence, Willa, Cicely, Maisie, Molly, Clover, Tana, Clementine, Thalia, Rose, Lily, Isis, Ella, Flora, Zara, Honor, Imogen, Lauren, Sophie, Martha, Holly, Rosanna, Ella, Amélie, Amber, Camilla, Rosie, Evie, Daisy, Breagha, Lilla, Kirsty...
If someone can get away with an Isla and an Isis and an Isla and an Ella, I reckon you can get away with an Isla and an Alice!