The Other Daughter — Lauren Willig

Raised by her widowed mother in genteel poverty in the 1920s in an isolated English village, for the past six years Rachel Woodley has been working in France as a nursery governess. When her mother unexpectedly dies, she returns to England to clear out the cottage, and finds a scrapbook full of cuttings from London society pages--all pictures of her supposedly deceased father, very much alive. He's an earl, socially prominent, with another daughter who is living a charmed life: a debutante, much photographed, and engaged to a rising Tory MP. Rachel's cousin confirms the horrible truth: her father is alive, with a legitimate, acknowledged family. Which makes Rachel...not legitimate. Everything she thought she knew about herself and her past--even her very name--is a lie.

Still reeling from the death of her mother, and furious at this betrayal, Rachel enters into an uneasy alliance with a mysterious man-about-town, who promises her access to her father. With his help, Rachel sets herself up in Roaring Twenties London under a new identity and insinuates herself into the party-going crowd of Bright Young Things, with a steely determination to unveil her father's perfidy and bring his--and her half-sister's--charmed world crashing down. Very soon, however, Rachel faces two unexpected snags: she finds she genuinely likes her half-sister, Olivia, whose situation isn't as simple it appears; and that Rachel herself might just be falling for her sister's fiance.