Lord Langcliffe's Lady by Donna Davidson (1998, Paperback)

Back in the 1300s a Langcliffe took a Thorne as his mistress and gave her a beautiful piece of land within his estate which would go to her male descendents forever. If there were only female heirs left, she must marry before her 25th birthday and her husband take the name Thorne or the land would revert back to the Langcliffes. Now it is the Regency era and Miss Thorne is nearly 25 and engaged to the local vicar. Lady Langcliffe had taken her son and left her husband years earlier because she said he gambled and had affairs with the Thorne sisters. Lord Langcliffe has been raised on his mother's terrible stories about the Thorne women and their immoral ways and his disreputable father. Now Lady Langcliffe wants Miss Thorne's engagement broken so that on the young lady's 25th birthday she will still be single and the land will go back to the Langcliffe's. Lord Langcliffe begins a campaign to destroy Miss Thorne's chances of marriage only something happens with the long time feud when he meets the enchantingly lovely Miss Thorne and her very moral aunts and begins to learn the truth about his mother's stories. Finally he must choose between his mother's truth and the Thorne's.