The Mésalliance by Stella Riley

At the beginning of the book, a young man meets an even younger woman and notes that she seems to be running wild. Eight years later, the man is the Duke of Rockcliffe and that young woman is the "spinster aunt" at her mother's sister. She isn't really any more beautiful but her acerbic wit catches his attention. So does the fact that her cousin is avidly chasing the duke. But when her cousin's schemes go horribly wrong, Adeline is the one who ends up being married to the duke.
I read a review of this book on Dear Author and thought it sounded good. And the first half was wonderful. But the second half suffered from So Much "if they had but talked to each other."

Three Stars
This book came out in 1990
Borrowed this book from Audible Escape
Opinions are my own