I know a few Charlottes who are always Charlotte, not shortened.
Harriets I know have sisters called:
Georgina
Emily
Charlotte
Grace
which I think are all perfect with Harriet
Other ideas:
Annabelle, Abigail, Arabella, Amelia, Alice
Caroline, Claudia, Cassandra, Clarissa, Caitlin
Daphne, Delilah, Daisy
Evelyn, Eloise, Emma, Eleanor
Florence, Frances, Felicity, Francesca, Freya
Georgia, Genevieve
Isabel, Isabella, Imogen, Isla
Juliet, Joanna, Josephine, Jessica
Katherine
Lucy, Lydia
Madeleine, Matilda, Miranda, Melissa, Margaret
Natalie, Natasha, Naomi
Olivia, Octavia
Philippa, Penelope, Phoebe
Rebecca, Rosalie, Rhiannon
Sophie, Sophia, Susannah, Sylvia
Tamara, Thea
Victoria, Vivianne, Verity, Vanessa
Xanthe, Zara