I am very happy now thanks to the flirtation between my sister-in-law and my ex-fiancé.

I will simply tell the former fiancé the truth. A little malicious sharp tongue? There's also a bit of comeuppance. The protagonist has their fiancé taken away by their sister-in-law. However, both the sister-in-law and the fiancé fell into decline on their own, so there wasn't really a satisfying comeuppance development. Yet one day, by chance, they run into their ex-fiancé at an unexpected place. And to him, who is now whispering sweet words out of nowhere, they decide to confront him with the facts. I am also posting this on Alphapolis.