She leaned her back against a fluted pillar and wondered if her brother was looking at the same stars tonight, wherever he might be. Do you see the white one, Quentyn? That is Nymeria’s star, burning bright, and that milky band behind her, those are ten thousand ships. She burned as bright as any man, and so shall I. You will not rob me of my birthright!
So I am confused a bit, Doran tells Arianne he was training her brother to be the heir because he intended her to be the next Targ queen- marry up and sib takes your place. But what about sideways matches, if Arianne successfully married Willas Tyrell- be Lady Tyrell and Princess? If their firstborn was a boy rule over Reach and Dorne? How do this consolidations of power work?
We don’t know. There is very little historical evidence in Westeros of an heiress marrying an heir.
The only example we have is in a much smaller domain, where two neighboring minor noble houses (House Webber and House Osgrey) merged their lands when the ruling lady of House Webber married the lone survivor of House Osgrey. (As much of the lands of House Webber had once belonged to House Osgrey many years earlier before their decline, this was a very satisfactory decision for him; and as she had needed to marry because her father’s will said the lands would go to a cousin if she didn’t, it was a mostly satisfactory decision for her.) Though notably this was about a hundred years ago, and neither house is mentioned in current Westeros, so we don’t know what the eventual fallout was.
Anyhow, that was a situation barely a step up from landed knights, just a matter of a few acres and two castles. When it’s a matter of Great Houses, and the control of entire regions of Westeros… we just don’t know what could happen. Especially in a situation like Dorne and the Reach, that are traditional enemies with many border wars… would it force a peace? Would there be arguments over whether the eldest child would rule over one land or the other? Or since they’re neighbors, would there be a merged kingdom, like Castile and Aragon? And how would the other Great Houses feel about this? (The land areas of Dorne and the Reach put together would be a little less than half the South.)
For what it’s worth, though… Littlefinger has arranged the betrothal of Sansa Stark, the lady-in-exile* of House Stark and the North, to Harry Hardyng, the future Lord of the Vale of Arryn. From what Littlefinger has said, he seems to think that the Vale lords will eagerly accept Sansa as their lady and also help her regain the North. (Whether he actually believes this is anyone’s guess.) It’s possible that in the next book we’ll find out the legal issues involved in a merge of Great Houses and kingdoms. Of course, I don’t expect this betrothal to work out, so it’s also possible we may not find out anything at all, unfortunately.
*(I know, Sansa’s actual legal title is complicated as hell, what with the North declaring themselves an independent kingdom, the Iron Throne attainting the Starks and granting the North to the Boltons, her believed-dead brothers that are actually alive but missing, the false Arya Stark that married Ramsay Bolton, and the legitimization of Jon Snow that’s potentially in Robb’s will — not to mention the fact that Sansa’s an accused-kingslaying wanted criminal — but that’s probably what the Vale lords would call her if she revealed herself to them, ok?)
And yes, the same issues of merged kingdoms were involved in the nipped-in-the-bud-betrothal of Willas and Sansa, though it’s uncertain if the Tyrells were planning on Robb’s death and Sansa becoming Lady of the North. As the Reach and the North are not even neighbors like the North and the Vale, I have no idea how this potential merge would have worked. And you may ask, what about Tyrion, isn’t he technical heir to the Westerlands, wouldn’t this have also been the same question? Well, Tywin had absolutely no intention of Tyrion inheriting Casterly Rock and was planning on making him the Protector of Winterfell instead, so the issues aren’t quite the same.
Anyhow. I hope the subject is discussed in future books (or even the World of Ice and Fire, some other historical example or something), because quite frankly I’d really like to know how it works myself.
Her father plucked up a cyvasse piece. “I must know how you learned that Quentyn was abroad. Your brother went with Cletus Yronwood, Maester Kedry, and three of Lord Yronwood’s best young knights on a long and perilous voyage, with an uncertain welcome at its end. He has gone to bring us back our heart’s desire.”
She narrowed her eyes. “What is our heart’s desire?”
“Vengeance.” His voice was soft, as if he were afraid that someone might be listening. “Justice.” Prince Doran pressed the onyx dragon into her palm with his swollen, gouty fingers, and whispered, “Fire and blood.“
Arianne gave him her most seductive smile. “We might share a bed together.”
Ser Daemon’s face was stone. “Have you forgotten, princess? I am bastard born.” He took her hand in his. “If I am unworthy of this hand, how can I be worthy of your cunt?”
She snatched her hand away. “You deserve a slap for that.”
“My face is yours. Do what you will.”
“What I will you will not, it seems. So be it. Talk with me instead.”
George R. R. Martin - The Winds Of Winter (online-published excerpt)