Arianne Martel, Doran's eldest daughter and heir of Sunspear, feared that her father's weakness would cause him to bow to pressures from the Iron Throne.  This possibly would include making her brother Quentyn heir rather than her.  Despite she being the eldest child.  So she enlisted the Sand Snakes and a few others in a conspiracy to put Myrcella Baratheon on the Iron Throne, as by Dornish law she should be queen.  She figured the resulting war (as she imagined a civil war would break out over whether Myrcella or Tommen should rule) would cause the people of Dorne to rally around her rather than Quentyn and she would secure her birthright.

But Arriane doesn't even exist in the tv series.  So it's all