An Unconventional Bride — Fenella Miller

An Unconventional Bride is the third in The Duke's Alliance series.
Mrs Mary Williams, a colonel's widow, arrives at Silchester Court with Miss Elizabeth Freemantle, who has been brought up as her sister. Beth is the Duke of Silchester's cousin and he is her guardian.
Lord Aubrey, the duke's youngest brother, finds himself designated to oversee the London debut of both Lady Giselle, his sister, and his lively cousin, Beth, as the duke is called away to his estates in the North.
Although Mary is only four years older than Aubrey she is more worldly and well-travelled. Mary is not thinking of marrying a second time, she values her independence too much, and certainly not to a young gentleman like Lord Aubrey.
Only when her reputation is lost, and marriage to Aubrey is impossible, does she understand that her feelings have changed.
Is it too late for them to find happiness together? Will the duke allow her to be part of his prestigious family?