Charlottes I know have siblings called:
Poppy, Stella, Chloe, Daisy, Antigone, Emily, Samantha, Amaya
Alexander X 2, Spencer, Hamish, Andrew, Matthew, Ben, Christopher, Aidan, Lucas, Jolyon
I think it goes well with any classic, conventional name really:
Abigail, Annabelle, Alexandra, Beatrice, Bryony, Daphne, Eleanor, Evangeline, Felicity, Francesca, Florence, Georgina, Grace, Hannah, Harriet, Isabelle, Isabella, Imogen, Jessica, Joanna, Juliette, Katherine, Katie, Laura, Lauren, Lara, Lucy, Madeleine, Matilda, Miranda, Melissa, Megan, Miriam, Natalie, Natasha, Naomi, Olivia, Phoebe, Rebecca, Rachel, Rosie, Sophie, Sophia, Sarah, Tamsin, Tamara, Victoria, Vanessa, Verity, Zoe
Alistair, Adam, Benedict, Daniel, Dylan, Elliot, Ethan, Frederick, Henry, Isaac, Jeremy, Jonathan, James, Nathaniel, Nicholas, Noah, Oliver, Oscar, Owen, Patrick, Reuben, Samuel, Solomon, Thomas, Vincent, William, Xavier, Zachary