What is the historical romance book where the girl convinces the man to let her marry him instead of her sister?

The historical romance book you're referring to is likely "The Bride" by Julie Garwood. In this novel, the spirited protagonist, a strong-willed woman, convinces a rugged Highland warrior to marry her instead of her more demure sister. Their tumultuous relationship evolves into a passionate love story set against the backdrop of medieval Scotland. The book blends humor, adventure, and romance, typical of Garwood's engaging style.