The part I'm fine with is actually the sister getting to run off with another man. The way I see it, the sister is going to sabotage her own future here. She's either going to play around and end up alone when he divorces her. OR he's going to be someone who will keep constant tabs on her and keep her from doing as she pleases. Either way it's not going to be a completely happy ending for her.

The part I'm not fine with is that the guy discovers the MC but just walks out and leaves her alone. I'd much rather that he not discover her and that he's forced to take the sister with him, only for her to basically end up in a very unhappy white marriage until he does find the MC.

I also would have liked to have seen MC's social status get fixed up, as people start to realize that they'd been tricked. I would have liked to have seen this stretched out into maybe another chapter or two. Eight at max, as there's no need to have a super long series.

Daisy goes back to her parents' house, lets her sister take her place as the Duke's wife, raises the nephew for 2-3 years while her sister lives it up as a duchess, only to cheat on the Duke (who knew quickly that Daisy had switched places with her sister all her life) and run off with a guy. So the Duke divorces the disappeared sister and starts going to see Daisy before revealing "Hey, I knew all along, I loved you since I was little and you told me the truth and I still want to marry you and take care of your nephew who everyone thinks is your son."