No Marriage of Convenience by Elizabeth Boyle

Mason St. Clair, the new Earl of Ashlin, has inherited a title for which there is no longer a fortune, thanks to his elder brother. Steeped in debt, with three ungainly nieces to marry off, Mason is desperate for relief. Only he doesn't expect it in the form of Madame Fontaine, a woman of questionable reputation. She arrives on his doorstep with partial payment on a debt owed to the former earl. When Mason demands full payment, she is at a loss. It's wacky Cousin Felicity who suggests that this woman, whom men cannot resist, can work off the rest of her debt by teaching the three wards how to attract worthy husbands. In a bind, Riley, as the Madame is known, agrees. Once the bargain has been struck, Mason finds that he too is falling under the Madame's spell, and it's not long before an additional couple is heading to the altar.