Their wedding is going to be so awkward. Everyone knows and sees them as siblings and one was already engaged and escorting that lady to parties and such, it's going to be embarrassing for her to lose the duchess position to a sister. It'll be weird for the family that looked after them to be at their wedding to, you turned down our son for your own brother? Lol. And how are the other two brothers going to act when they confess their love, as far as they knew all of them were just siblings to each other now suddenly your older brother is making out with your little sister. Realistically high society would be all over them, and even their children wouldn't be able to escape the gossip about them as well, duke family or not.