The Crown Prince said he wanted to break off his engagement because he found true love, but then I was told that his destined love interest was my younger sister, and she was carrying the Crown Prince's child! So what exactly was the revenge I took on those two?

The prince, having awakened to true love, declared the annulment of our engagement, and I fell into despair upon discovering that his new love was my younger sister—the one who always causes me trouble—and that she was pregnant. My parents tell me to bless this union, and fine, I shall bless them. In appearance, at least. So what is my plan for revenge?