Enlightening Delilah - 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.

Stunningly beautiful, the dazzling Delilah has had plenty of marriage proposals but having once been spurned by Sir Charles Digby, the only man she has ever loved, she is now a hardened heartbreaker who toys with all her suitors.
And so it is up to the eccentric Tribble sisters to teach her the meaning of true love again.