My fiancé's toxic parents are trying to nullify our engagement.

My fiancée has parents who dote excessively on her younger sister. In time, my fiancée's younger sister became a "greedy sister" who wanted everything her older sister had. This is a story that takes place after my fiancée's "greedy sister" set her sights on me, and I decisively cut ties with her. You think I'd just allow my fiancée to unilaterally call off our engagement like that? *Also published on Alphapolis and Kakuyomu.