I know sisters called Willow and Autumn but I guess if you've got a Summer, Autumn is probably a bit much.
It doesn't have to be nature themed, of course, but I agree that it be lovely. Robyn and Hazel would be perfect but obviously no good if your husband doesn't like them. :(
For nature-y ideas: Willow, Summer and ...
Aspen, Aven, Briar, Calla, Coral, Dahlia, Daisy, Eden, Fern, Fleur, Holly, Honey, Ivy, Iris, Juniper, Jasmine, Jonquil, Lily, Misty, Meadow, Nova, Olive, Ocean, Primrose, Rowan, Rose, Raine, Raven, River, Sorrel, Saffron, Star, Wren, Violet
For other kind of modern-popular names like Willow and Summer:
Ava, Aria, Bella, Clara, Ella, Eva, Freya, Grace, Harper, Hallie, Isabelle, Layla, Madison, Maia, Mia, Nina, Paige, Phoebe, Ruby, Sienna, Savannah, Tilly, Zara
For other kind of hippy/quirky/flow vibe but not specifically nature ideas:
Aphra, Amaya, Bronte, Blue, Calista, Celyn, Cleo, Danica, Delphi, Ebony, Everly, Effie, Etta, Enya, Fallon, Giselle, Harmony, India, Inez, Isolde, Juno, Jacinta, Kiki, Livia, Lorelai, Leonie, Liberty, Melody, Mika, Noa, Nyah, Piper, Ramona, Saskia, Serena, Serenity, Sephorah, Tatiana, Talitha, Tamara, Verity, Vashti, Xanthe, Zaryn, Zillah, Zelda