Marrying Harriet - M. C. Beaton

'If you have a Wild, Unruly, or Undisciplined Daugher, two Ladies of Genteel Birth offer to Bring Out said Daughter and Refine what may have seemed Unrefinable.
We can make the Best of the Worst'

When Amy and Effie Tribble, two charming but impoverished spinster sisters, lose out on an inheritance, they place this advertisement in The Morning Post and hire themselves out as professional chaperones.
Vowing to prepare even the most difficult misses for marriage, the Tribble sisters will spend a London season on each client, educating them in their School for Manners.

Miss Harriet Brown, daughter of a Methodist minister, is the embodiment of propriety and Christian charity - too much so, perhaps, for her own good.
The virtues Harriet possesses are far from fashionable but the Tribble sisters feel confident their new charge will attract a worthy vicar or two before the end of the season - if first they can vanquish confirmed rake and gambler Lord Charles Marsham who seems perversely determined to woo Harriet!