Wicked Pleasures By Penny Vincenzi

A contemporary saga of two dynasties who find themselves helplessly entwined when a brother and two sisters discover that they all have different fathers: none of them Alexander, Earl of Caterham, who was married to their mother for almost twenty years. The discovery of this lie at the heart of the family sets in motion a chain of events which will change their privileged lives forever, bring to the brink of ruin the great family banking business on which their fortunes are based, and awaked some very dangerous and conflicting ambitions...