I have a Savannah so obviously love it but do think it's too similar to Sienna. Though I only really think that because of the S sound I think and, judging by your boy choices, you're quite keen (or at least unbothered) on alliteration so maybe Sienna and Savannah would work. If you're put off by the shade of brown you could use Savannah instead of Sienna?
Or, if you stick to Sienna and do want alliteration - Sienna and:
Saffron, Sorrell, Saskia, Cecily, Skye, Scarlett, Summer, Cecilia, Celeste, Sutton, Sylvia, Susannah, Samantha, Sapphire
Or, if you don't really want the 'matchy' thing - Sienna and:
Avery, Aubrey, Alyssa, Alexis, Abigail, Bryony, Bethan, Chloe, Cassidy, Callie, Charlotte, Devon, Darcey, Eden, Erin, Effie, Fallon, Georgie, Grace, Halle, Harper, Hope, India, Jade, Jasmine, Jessica, Jordan, Katie, Kristen, Katrina, Lacey, Lexi, Madison, Macey, Mika, Nicole, Nia, Natalia, Paige, Piper, Poppy, Quinn, Riley, Ruby, Romilly, Taryn, Tansy, Tia, Vivianne, Willow, Zia