Lord Hugo's Bride — Marina Oliver

Lord Hugo, Earl of Winton, needs a wife and an heir if his weak cousin William and his unpleasant French wife Hortense, with the son who might not be her husband's, is not to succeed him.

He decides Phillipa Stannerly will be suitable. She is pretty, and an heiress. Phillipa, in love with Charles, is distraught.
Then her French cousin Sophie du Plessis comes to London and promises to help her. Their mothers were twins, and the girls are sufficiently alike to confuse people who do not know them well.

When Hugo sees Sophie on her own he offers her a lift home, intending to pursue his suit in the belief she is Phillipa. She maintains the deception, hoping to prove to her uncle that Hugo is not a fit husband for Phillipa.
Her behaviour is, however, unlike Phillipa's shy response, and Hugo is intrigued.