The Bride Says No (The Brides of Wishmore 1) by Cathy Maxwell

Lady Aileen never thought she’d be in this position—falling in love with her sister’s betrothed! Of course, her sister did run away rather than marry him, and he doesn’t seem to be all that interested in her sister either, but even so, such a match is impossible...isn’t it?

Aileen is certain she must turn away from the man she loves in order to protect her sister’s reputation. However, he’s not one to give up without a fight, and now that he’s met a woman he wants to marry, nothing on earth could convince him to let Aileen go.