I think the prince used to be Estien, like her father adopt him in the past.... that is why he is not surprised....
the flashback wouldnt have enough weight to support your theory since if the queen was careful and scheming, it would be impossible to hide the prince under Estella's family for long without Estella knowing since she and her father had a very close relationship, having been the only ones together in the remainder of their lives and especially that they are only a small family with little influence, the queen's eyes and ears would easily catch on. (But ig this last bit's questionable since nobody questioned Estien's appearance when he came out of the wood works lol) The father, as well, would not be able to train the prince with the current abilities the prince has which seems more aligning of those mercenaries he was with in his youth.
I think its more plausible the prince suspects the identity of this other sibling and has a hunch that Estella is hiding her true gender which he is taking advantage of, knowing theyre a small house but maybe you might as well be predicting the future, who knows HAHAHAHA.




There is communication in this story! Really like the end <3