An Advantageous Marriage — Alice Chetwynd Ley

HIGH-SPIRITED, SHARP-WITTED, SHE PLAYED A GIRL'S DEFIANT GAME TO HIDE A WOMAN'S LOVING HEART.

SHE COULDN'T LET MARRIAGE SPELL THE END OF LOVE!

The ambitious Turvilles remembered little cousin Eugenia as a plump, vulgar, provincial girl--but she had come into a fortune and it was up to Frank or Aubrey to woo and win her. Mischievous, canny, grown into all extraordinary beauty at 16, Eugenia fathomed their scheme and determined not to disappoint hem. She had plans of her own. She would choose her own man...a man like Sir Peter Martyn. But when sophisticated, widowed Lucilla arrived from Regency London to reclaim Peter for herself, and Frank penetrated Eugenia's disguise to find he truly cared, her clever ruse threatened to backfire, costing her the only love that could bring her happiness.
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Corgi (UK) Edition:

When Baron Turville's brother married into Trade, the connection was deplored - and shunned. Nevertheless, years later, the family graciously invited the orphaned daughter of the marriage to Lydeard Hall. For Eugenia heiress of her Yorkshire grandfather, was about to make her London debut... The unknown country bumpkin - however unappealing - could prove a wealthy catch for Adrian or Francis, her Turvill cousins...

But even before her arrival the enchanting Eugenia was secretly apprised of the family's scheme, and mischieviously resolved to fulfil their expectation of 'a vulgar Yorkshire Miss'. A masquerade which afforded her a deal of sport - until a third romantic suitor made his appearance...