Ruthless by Anne Stuart
Ruthless (2010) is the first entry in Anne Stuart's House of Rohan historical romance series.
Elinor Harriman is intelligent, loyal, very practical--and also penniless and trying to keep the household, including her lovely younger sister, afloat while dealing with a mother addicted to gambling and men. (Not much in the way of mother-daughter love in this story, but the two sisters have a strong, supportive relationship.)
Enter Viscount Rohan, jaded by life and having the money and dark good looks to get anything (or anyone) he wants. Leader of a group of English aristocrats living in Paris during the mid-1700s, he seems to live only for his next moment of debauchery.
But then he finds the wonderful Elinor, and the seemingly unequal couple is actually quite well-matched in temperament and intellect. And Elinor actually needs the kind of assistance only someone like Rohan can supply when matters in her extended family come to a head.
Excerpt from Kindle Location 1327-1341:
"And I've just met your lovely sister. . . "
She moved with astonishing speed, somehow managing to come between him and the pretty little chit whose hand he'd been holding, and everything suddenly made sense, which pleased him. He preferred his life with a certain order, and anomalies, while entertaining, needed to be explained, before one could move on.
Though the anomaly that was Elinor Harriman was going to take a bit longer to figure out.
"Thank you so much for coming," she said, her voice brooking no opposition. "You've been extremely kind, but we wouldn't think of keeping you from your guests."
She was tall enough, and solid, but he could simply pick her up and move her out of the way if he had any interest in the pretty little sister.
He had not. He'd had enough pretty girls and beautiful women to last him a lifetime. The other one, however, was proving interesting. He was still aroused from their time in the carriage--if she hadn't pretended to fall asleep he would have had her skirts up over her head in a minute.
You can learn more at the author's site here. You can also download the free e-book prelude to the House of Rohan trilogy, The Wicked House of Rohan, at the site. If you don't enjoy the relatively short prequel piece, there isn't much point in reading the series.
My overall personal rating of Ruthless is a B.