During the previous 22 chapters I didn't care when they said they were siblings but currently in chapter 23 all those girls tried to be with him but he was only looking at "his sister"
If I understood correctly, FL found her real biological brother drunk in chapter 18 since they called him the son of Duke Xu and if I remember correctly in the comments they said she was his daughter
If we look at the timeline we are currently in what would be adopting the protagonist, then the exam, maybe getting some honor of his own, and then it is discovered that the girl is not his sister, then the personal chaos and he marries her.