Rebel Heiress by Jane Aiken Hodge
Henrietta was no one's enemy. She came to London from Boston in search of her father-- a father thought lost to her until she discovered her aunt's treachery. Now armed with her mother's marriage papers, she set sail for England to prove her identity and, unknowingly, to gain a fortune--and a love--she had never dreamed of. Henrietta was too beautiful to ignore. Her wealthy father was enchanted with her. Her stepmother hated her on sight. And her stepbrother Cedric was an enigma.