I reread Arianne’s chapter from AFFC where she and her companions ride with Myrcella. The group of seven include Myrcella Baratheon, Arianne Martell, Arys Oakheart, Andrey Dalt, Sylva Santagar, Garin of the Orphans, and Gerold Dayne. Areo Hotah shows up and Arianne’s scheme is ruined. Later in the book, she wonders who betrayed her.
I posted the following years ago…
Fourth, there is some information regarding Arianne's traitorous friend... Andrey Dalt, Garin, or Sylva Santagar. Drey and Garin were both exiled. Sylva was wedded to old man Estermont. Remember that Arianne is her father's heir because the laws of succession in Dorne allow for the eldest child to inherit regardless of sex. Soooo, Sylva is supposedly being punished by marrying an old, old man. When he dies childless, who will inherit? Sylva. Oh, did I mention that she's an only child so that she'll inherit her family's lands and titles as well? It seems to me that she now stands to inherit both Spottswood and Greenstone. Two were exiled and one was gifted with lands and title... do we still wonder who the traitor was?
Since then, I think someone showed me a family tree, or some information that showed that Sylva would not inherit Greenstone when her husband dies. So I reevaluated my assessment of the traitor. But reading this chapter tonight leads me again to think that Sylva was the traitor.
In this chapter, once Hota appears, Martin writes the actions of all the characters except for silver. Myrcella sits Garin raises his hands in surrender. Andrey unbuckles his sword belt and throws it away. Gerold yells at Arys to not attack. Arianne is dumbfounded and cannot speak. Arys charges forward. Arianne found her voice and shrieks “Nooo”. Arys is wounded. And then the following happens…
“
No,” some girl was shouting, some foolish little girl, “
no, please, this was not supposed to happen.” She could hear Marcella shrieking too, her voice shrill with fear.
This chapter is full of Arianne’s thoughts in italics. And yet she hears some foolish little girl in italics. And Sylva is the only one not mentioned doing this encounter. Did Martin forget about her as he did with Bran’s omission during the feast for Robert at Winterfell? Or is Sylva the one shrieking “this was not supposed to happen”?
Though she has not mentioned, it is easy to imagine Sylva sitting next to Myrcella.
Neither Sylva nor Arianne is that mature, and both could be seen as the stupid little girl. But it seems to me from the text that it is most likely not Arianne. Therefore Sylva is the traitor.