I Heard My Lovely Sister-In-Law's Engagement Was Broken So I'm Going To Show Them My 'thanks!' summary:

The most talented girl in the kingdom, Roselia, is a scholar, fighter, and the apple of everyone's eye. Today happens to be the day of marriage to a childhood friend and now lover, Robert, the head of a ducal family. Everyone was congratulatory and enjoying the wedding, until...Robert's cute little sister Margaret was brought into the ceremony in tatters! Margaret was attending the Royal Academy and engaged to the prince. But he broke the engagement accusing her of a crime that was never committed, and ruthlessly beat her! Roselia is now furious at their disgusting treatment of Margaret, who did everything to clean up the prince's messes! But is Roselia really going to confront this corrupt royal court head on?!