I think Sienna is too similar to Selina but Savannah isn't. Unless you prefer to avoid alliteration.
Other ideas:
Adeline, Amber, Astrid, Annika, Bryony, Bronte, Clarissa, Cassandra, Carmen, Camilla, Daphne, Emmeline, Evelyn, Evangeline, Erica, Frances, Gabrielle, Georgia, Harriet, Imogen, Jessamy, Joanna, Jasmine, Kathryn, Lois, Livia, Madeleine, Melody, Natasha, Nicole, Nadia, Octavia, Pearl, Phoebe, Rebecca, Rosanna, Robina, Saskia, Sorrell, Tamara, Tamsin, Verity, Vanessa, Victoria,