Never Forget Love (Camfield #29)

Far from the dazzle and extravagance of Regency Society, Nerissa Stanley and her brother lead a quiet country life as she keeps house for her father, a distinguished but improverished author.
Suddenly their elder sister Delphine arrives. Some years before she had managed to marry a rich, elderly Peer and has since ignored her poor relations.
But now she needs their help. Recently widowed, she is hoping to marry the Duke of Lynchester, but has to establish the suitability of her own family. She offers her brother and sister a generous and badly-needed sum of money if they will pretend to be the servants when she brings the Duke to meet her father.
Reluctantly they agree but the scheme is uncovered by chance and the Duke invites them all to joina a House Part at his superb Elizabethan house.
Enchanted if a little overawed by the magnificence of his establishment, Nerissa is nonetheless able to lift the family curse that blights the Lynchester family. Meanwhile, the Duke, bored and cynical about the Society beauties who pursue him, finds in Nerissa a natural, true loveliness. In spite of Delphine and many difficult and overwhelming obstacles - love wins.