The Fabric Of Love (Harlequin Regency Romance Series 2 #58)
Elizabeth Michaels

Overnight, Katherine L'Ecusson, daughter of a shopowner, became known as Katherine Crest, the new darling of the ton.
Unbeknownst to her, Lord Ashington had agreed to stand as a brother and protect her from harm. But after a time, his feelings for her were anything but brotherly.
Katherine was in a coil. She did not wish to discourage his attention, but if she did not, they would both be ruined.