I'm breaking my engagement with you and getting engaged to my half-sister instead? Very well then, you're banished!

I was raised as the only daughter of a viscount household and its heir. And yet, it's been several years since my mother passed away. Just recently, I gained stepsisters. Yes, new stepsisters... and apparently a half-sister at that. It seems that several years after my mother's death, my father declared that enough time had passed for mourning. Once things had settled down, he brought in his mistress—a woman he'd been keeping even before his marriage to my mother—and married her. Furthermore, he's now officially recognizing my half-sister and adding her to our viscount household registry. And to top it all off, it was all announced after the fact!? To put it mildly... he's been betraying my mother this entire time, even before their marriage. He's seriously a worthless piece of trash, that damn old man!! Oh my, my language has become rather crude. My apologies. That damn old man is skilled at self-preservation and cunning besides being trash. Unlike what often happens in stories, our household isn't based on my mother's bloodline—it's simply the damn old man's bloodline. So there's no guarantee my position as heir won't be overturned at any moment. What's more, my half-sister is apparently a legitimate daughter of that damn old man. I'm pretty much doomed, aren't I? My half-sister began filling my fiancé's—that empty-headed, thick-skulled moron's—head with delusions. And then, at a party during the noble academy exchange meeting—— "I'm breaking my engagement with you and want to become engaged to my true love, the one I'm truly connected to!" He actually went and did it! "You're breaking your engagement with me to become engaged to my half-sister? Very well then, you're banished!" Suppressing the urge to laugh mockingly, I responded in high spirits. The setting is intentionally vague. Published on AlphaPolice.