Say You Love Me by Johanna Lindsey.   
Lord Derek Malory purchases at auction a mistress named Kelsey Langton, but he soon realizes there is more to the lovely but penniless young woman than beauty as she has grace, wit, and intelligence as well.  Her parents' deaths left Kelsey Langton penniless and responsible for the safety and well-being of her younger sister, Jean. Now circumstance and human frailty have conspired to leave them homeless as well, unless Kelsey can avert the disaster. And he never expected that he would fall in love with her.