Bath Tangle by Georgette Heyer
The Earl of Spenborough always had been noted for his eccentricity. Leaving a widow younger than his own daughter Serena was one thing- Leaving his fortune to the trusteeship of the Marquis of Rotherham - the one man the same daughter had jilted - was quite another. In a tangle of marriage and manners the like of which even Regency Bath has rarely seen, Lady Serena finds herself involved with her lovely young stepmother, Lord Rotherham and her own childhood sweetheart.