@jinx I read the raws. Because a few chapters are in English I can my brain can understand enough to read some chapters in.
By the way, to everyone who's curious. Either it was lost in translation or not put in the manga, the novel explains that the Prince does swap because the request came from the Duke and he thought that this would result in the Duke owing him a favor. Also, the poorly explained part, is that the Prince believed that because they were sisters that they would be raised the same there fore be similar (the manga only states that they are similar because they are sisters, thereby skimping on some details).
Overall, I'd put 35% blame on the Prince for lack of info gathering and hasty/poor planning, 60% blame on the Duke for his caving to the little sis and 5% blame on the FMC for not standing up for herself and fobbing off responsibily of the decision to someone else.