At last comes love / Mary Balogh.

Enhanced descriptions from Syndetics:

Step into a world of scandal, intrigue, and enthralling passion as New York Times bestselling author Mary Balogh sweeps us into the lives of an extraordinary family: the Huxtables. Margaret, the eldest, embarks on the most risqué adventure of her life and agrees to marry the most notorious man in London . ...

Only desperation could bring Duncan Pennethorne, the infamous Earl of Sheringford, back home after the spectacular scandal that had shocked even the jaded ton . Forced to wed in fifteen days or be cut off without a penny, Duncan chooses the one woman in London in frantic need of a husband. A lie to an old flame forces Margaret Huxtable to accept the irresistible stranger's offer. But once she discovers who he really is, it's too late--she's already betrothed to the wickedly sensual rakehell. Quickly she issues an ultimatum: If Duncan wants her, he must woo her. And as passion slowly ignites, two people marrying for all the wrong reasons are discovering the joys of seduction--and awaiting the exquisite pleasure of what comes after....