Duchesses Don’t Cry by Barbara Russell

Anthony Beaufort, Duke of Gloucester, is in need of a wife, and his grandmother is in charge of choosing the perfect lady for him.
But when he meets Helen, he finds her lack of personal opinions and her affected manners too daunting for his taste. Instead, Isabella, Helen’s sister, impresses him with her laughter and spontaneity.
Clever, spirited, and beautiful, Isabella will be a great duchess, someone he could rely on and trust to share his work with.
His grandmother disagrees. Isabella is a romping young woman with delusions of adequacy. Not to mention she supports women’s suffrage.
Isabella enjoys Anthony’s company, but marriage isn’t her priority, and Helen’s biggest wish has always been to marry a duke.
How could Isabella hurt her own sister? But how could she refuse a duke?