Lord of scoundrels / Loretta Chase.

Enhanced descriptions from Syndetics:

They call him many names, but Angelic isn't one of them . . .

Sebastian Ballister, the notorious Marquess of Dain, is big, bad, and dangerous to know. No respectable woman would have anything to do with the "Bane and Blight of the Ballisters"--and he wants nothing to do with respectable women. He's determined to continue doing what he does best--sin and sin again--and all that's going swimmingly, thank you . . . until the day a shop door opens and she walks in.

She's too intelligent to fall for the worst man in the world . . .

Jessica Trent is a determined young woman, and she's going to drag her imbecile brother off the road to ruin, no matter what it takes. If saving him--and with him, her family and future--means taking on the devil himself, she won't back down. The trouble is, the devil in question is so shockingly irresistible, and the person who needs the most saving is--herself!