"Dornish law does not apply." Tyrion had been so ensnared in his own troubles that he'd never stopped to consider the succession. "My father will crown Tommen, count on that."

"He may indeed crown Tommen, here in King's Landing. Which is not to say that my brother may not crown Myrcella, down in Sunspear. Will your father make war on your niece on behalf of your nephew? Will your sister?" [Oberyn] gave a shrug. "Perhaps I should marry Queen Cersei after all, on the condition that she support her daughter over her son. Do you think she would?"

Never, Tyrion wanted to say, but the word caught in his throat.... "I don't know how my sister would choose, between Tommen and Myrcella," he admitted. "It makes no matter. My father will never give her that choice."

"Your father," said Prince Oberyn, "may not live forever."

Something about the way he said it made the hairs on the back of Tyrion's neck bristle. Suddenly he was mindful of Elia again, and all that Oberyn had said as they crossed the field of ash. He wants the head that spoke the words, not just the hand that swung the sword. "It is not wise to speak such treasons in the Red Keep, my prince. The little birds are listening."

"Let them. Is it treason to say a man is mortal? Valar morghulis was how they said it in Valyria of old. All men must die. And the Doom came and proved it true."

George R.R. Martin -- A Storm of Swords