I recommend " After Married to a Traitor, an Ugly Woman Becomes a Regent Princess " to you. This novel fits the information you provided. When the female protagonist first transmigrated, she was very ugly. Her sister was jealous and had no choice but to marry the Regent. The male lead was the Seventh Prince and had always loved his sister. However, after spending time with the female lead, he developed feelings for her. I hope you like my recommendation.😘