The Guardian Duke by Jane Charles

With the death of his uncle, Henry Simpson, Duke of Eldridge, became guardian of his five female cousins. Given that they were raised by a strict vicar who allowed them no freedom whatsoever and forced pious prayer while warning that London was full of sin, Eldridge expected that they would wish to quickly wed and settle into stable domesticity after experiencing one Season. At least that was his hope until they fell under the influence of a most independent lady.
Having reached her majority, and able to enjoy certain freedoms that are only allotted spinsters, Lady Penelope Johansen is in no hurry to wed. She has also learned much in her five and twenty years and when she meets the five sheltered sisters, new to London and enjoying their first Season, Penelope takes it upon herself to make certain they know which rules to follow, which rakes to avoid, and how to enjoy their first Season to the fullest. All would work out perfectly if not for their overbearing guardian.

If only Lady Penelope would assist in seeing his cousins married instead of encouraging independence, Henry would welcome her interference. Instead, her presence is a torment and she does not like him very much.