User blog comment:Observer Supreme/Rebecca's lineage/@comment-124.171.165.244-20180626235454

Maybe not all but most of them knew before Doflamingo's coup. They knew because they had likely seen Kyros, Scarlet and later Rebecca around the place, buying food and selling sunflowers. (I think that's what they did) They probably would have also seen King Riku and his family occasionally visit them. Because even if they went in disguise the guards where still following them.

But like with the Dwarfs the Dressrosans just went along with it. So it was an open secret that Kyros and Scarlet had gotten married and where living together and Scarlet had faked her death to do this.

Then along comes Doflamingo. The coup happens and the surviving people who aren't turned into puppets, put in the coliseum or killed are the ones who didn't rebel against Doflamingo and they can't remember or saw what happened to those who did. This impounds on what the army did that night not realising that Doflamingo was controlling them. So they start hating the royal family and those that don't pretend they do because they're terrified of what might happen if they don't do what Doflamingo wants.

Eventually Rebecca gets captured and put into the Coliseum. Doflamingo knew who she was because Monet had been working in the palace for an unknown amount of time before the coup, and some of the people who knew who she was now hated her guts for what they thought her grandfather did. So it wouldn't be long before everyone knew, as I'm sure that as she was entering the Coliseum for the first time she was introduced as the granddaughter of King Riku.