I saw a Facebook friend complained about how Ellaria Sand's treason is silly and implausible. Personally I think they're doing it to show that the Dornish king's mercy (is he a king?) will eventually be his downfall. Bastards, incest, whatever, no one is special in Dorne, almost nothing looked down upon, and they've seemed to not have the same political in-fighting the other kingdoms have had. Which makes them complacent. Ellaria is going to implode their serenity from the inside. Just my guess. Edit: I mean, even the peaceful gardens seems to be a non-subtle clue.
Or she dies stupidly and none of it matters. Who knows with this show at this point.