I Was Robbed of My Fiancé by My Younger Sister, and Now I'm Marrying a "Blood-Thirsty Mad Dog"

"You are not fit to be queen." The First Prince discarded those words and unilaterally broke off our engagement. It seems my feelings—the support I gave behind the scenes of his administration and the loyalty I devoted—never reached him. Taking my place as his fiancée was my younger sister, who had nothing but her cute appearance and a talent for manipulating men. The prince became infatuated with my sister, and I was driven out of the castle. With no place for me in the capital or my family home, I ended up under the command of a border knight commander feared as a "blood-hungry mad dog." He's supposed to be rough, taciturn, and dangerous... yet for some reason, he carefully brews me tea every morning? In this peaceful life, I finally became able to spend my days without needing to be useful to anyone. And in the capital, those two who betrayed me are crashing down with a resounding noise. The First Prince? My sister? ――Ah, come to think of it, such people existed, didn't they?