I like Thea. I don't think sibling names have to 'go' but I might worry about the ph and th sounds. For a child who might already struggle with th, Thea might become 'fee-a' when given the added confusion of 'fee-bee'.
Thea Evangeline
Thea Antonia
Thea Madeline
Or, Phoebe and:
Abigail, Annabelle, Bethany, Beatrice, Charlotte, Cassandra, Darcey, Eleanor, Evelyn, Felicity, Freya, Georgina, Grace, Hermione, Harriet, Isabelle, Imogen, Jessica, Joanna, Katherine, Lydia, Lorelei, Miranda, Matilda, Natalie, Natasha, Olivia, Rhiannon, Rebecca, Summer, Scarlett, Tamara, Tegan, Victoria, Verity, Willow, Zoe, Zara