Didn't George said that Brienne is actually descended from Dunk? Not narratively wich she is but by blood.
The only descendant of Dunk officially confirmed by GRRM is Brienne of Tarth, my favorite candidate to be Sansa’s true knight.
GRRM has stated that readers have met a descendant of Dunk in ASOIAF:
But I did get up the courage to blurt out one small SoIaF query, which was if we’ve met a descendent of Dunk’s in the SoIaF. He said yes.
[…] I asked GRRM if we’d met one of Dunk’s descendants in the SoIaF, and he said Yes. I didn’t ask him who, because I’d rather find out by reading or reason, only I’m lousy at the reasoning part and want to make you guys do it for me.
He later stated that he gave a strong hint to Dunk’s descendant in A Feast for Crows:
Asked if he’ll ever tell which character is Dunk’s descendent. Got a rather acerbic, “I gave a pretty strong hint in the new book,” to which I sheepishly replied “Yea, but I read it real fast, in three days.” I told him I suspected Brienne but thought that she was too obvious and that he’d be more subtle than that and he said, “You think?” Coy bastard.
Given that in A Feast for Crows, Brienne recalls finding a shield with Dunk’s sigil in the armories of Tarth, it was widely speculated but not confirmed (yet) that she is descended from Duncan the Tall
The arms of Tarth were quartered rose and azure, and bore a yellow sun and crescent moon. But so long as men believed her to be a murderess, Brienne dare not carry them. “Your door reminded me of an old shield I once saw in my father’s armory.” She described the arms as best she could recall them.
The woman nodded. “I can paint it straightaway, but the paint will need to dry. Take a room at the Seven Swords, if it please you. I’ll bring the shield to you by morning.”
[…] The captain’s sister found her in the common room, drinking a cup of milk and honey with three raw eggs mixed in. “You did beautifully,” she said, when the woman showed her the freshly painted shield. It was more a picture than a proper coat of arms, and the sight of it took her back through the long years, to the cool dark of her father’s armory. She remembered how she’d run her fingertips across the cracked and fading paint, over the green leaves of the tree, and along the path of the falling star.
—A Feast for Crows - Brienne II
However, in 2016 at Balticon, Martin confirmed Brienne’s descent, stating that the exact relationship between Duncan and Brienne would be “revealed in time.”
At BaltiCon, fan Kristen Reed Treado asked, “Will we ever learn how Brienne descends from Dunk?”
GRRM replied, “Eventually. All will be revealed in time.”
Brienne is already Sansa Stark’s sworn sword. She swore her allegiance to Sansa’s mother, Catelyn Stark, and made an oath to find Sansa Stark. Brienne also wields Oathkeeper, a sword made of Ice (House Stark ancestral sword) and a repainted House Lothston (Harrenhal/Sansa’s maternal lineage) shield with the arms of Ser Duncan the Tall (a shooting star above an elm tree on a field with the color of sunset). Brienne commissioned the re-painting of the Lothston shield with Ser Duncan’s arms while being in Duskendale (Jonquil Dark, Dontos Hollard). A full circle enclosed around my favorite candidate to become Sansa’s true knight.
Even before the confirmation by the author, it was clear that Brienne was deeply connected with Dunk and somehow following his steps. You can find more parallels between Brienne and Dunk here and here.
So, if Brienne somehow ends up in the Vale of Arryn in time for the tourney at the Gates of the Moon, mysterious knight style, and enters the lists, competes, wins and crowns Sansa. Or if she somehow interrupts or shortens the tourney and there is a trial involved that she will fight in order to defend Alayne Stone/Sansa Stark, I will be more than pleased. The same if Brienne at some point rescues Sansa from anyone that may hurt, abduct or imprison her. Sansa deserves agency and freedom and good persons around her and Brienne deserves to fulfill her oath and become an appreciated and valued knight.
Sansa, though … I will find her, my lady, Brienne swore to Lady Catelyn’s restless shade. I will never stop looking. I will give up my life if need be, give up my honor, give up all my dreams, but I will find her.
—A Feast for Crows - Brienne II
Brienne is a worthy representative of her ancestor Ser Duncan the Tall, both knights that remember their vows. The truest kind.
Finally, it’s worth mentioning that the author has described Brienne as “Sansa with a sword.” This statement confirms that these two characters have an important interrelation.