Your tastes seem quite similar, you've just picked different favourites.
If you both like Sienna, could you not go for that?
Otherwise, I'd say Thea, Brooke, Olivia, Cassia, Poppy and Isla are in the same camp as:
Amelia, Alice, Alexa, Amber, Bryony, Charlotte, Chloe, Connie, Darcey, Emily, Eleanor, Ella, Eliza, Faith, Flora, Freya, Grace, Georgia, Hannah, Harriet, Imogen, Jessica, Jasmine, Katie, Kathryn, Katelyn, Lily, Lucy, Leah, Matilda, Madeleine, Natalie, Paige, Pippa, Phoebe, Ruby, Rosie, Sophie, Savannah, Tegan, Tessa, Willow, Zara, Zoe
My favourite of your joint boys list is Zavier if it spelt Xavier. Otherwise, I like Reuben. Theo, Lewis and Inigo also good.
As alternatives, how about:
Aidan, Christian, Dylan, Elliot, Elijah, Felix, Harvey, Harley, Hugo, Isaac, Josiah, Luca, Milo, Micah, Noah, Oscar, Patrick, Ruan, Rory, Solomon, Teo, Vincent, William, Zachary