Considering the Prince was 21 (I know she mentions it, but I forgot. But he's 5 years older than the MC who was 16 at that time, so yeah 21), yeah.
That would also be a 7 year age difference between him and his little sister.
And from how they made it sound, the Prince got engaged to the MC with so far the main intention to cover up this..... relationship with his sister, and that was when the Prince was 16, we all know the MC was (well is technically since they went back in time) 10, and the Princess is two years younger than her which would make her 8 at this point in time, and this comes with the implication that this has been going on for a much longer period of time as well. And considering how easily the Prince manipulated and took advantage of a 10 year girl until she was 16 and all the stuff she went through for him, it wouldn't surprise me if he had been grooming and manipulating his sister since they were little. It wouldn't surprise me if she had no illness and he just made her think that she does, because he doesn't want her going out and risking other people becoming interested in her (or even her to them) and for her to realize that what has been going on between them wasn't right in the slightly, and he wants to keep hee all to himself.
But in conclusion, yeah, he's a freaking pedo with a sister complex.
ヾ(❀⁰▿⁰)ノ ︵ ┻━┻ ︵ ╯(°□° ╯)
( ^ All of us who just wanna throw a table at this jerk. )
We all know that prince Gerald had an affair with his blood related sister and so that's why he wanted to kill the female lead, right? But what if this prince was forcing his little sister to have an affair with him? That would be considered rape right? I don't blame the female lead for not considering this fact but I hope this would be discussed further in the future.
There are other conclusion I've thought :
1. Either the prince or the princess (his little sister) is bewitched or poisoned to do stuff
2. That is not his blood related sister maybe that's his step sister or half sister (who knows)
Or he is just a bastard with a sister complex/ siscon/ pervert and that he and his sister are perfectly aware what the Fuck they are doing.
Thank you for reading this.