20 years after the end of the Dragon Rider war, Birgit from Carvahall made an encounter with wandering spirits after a fight with one of her twin sons.

When she regain consciousness, she's back at 20 years ago, in a loving and content family with her beloved Quimby. It was a month before the come of the Traders.

However, with all the knowledges she earned from another lifetime.

After a talk with Heimir to prevent future fight, she decided to change the story that would become if the dragon egg does hatch for Eragon again. Or if she stay at Carvahall.

First, she wanted to move away from the village, to live peacefully and out of the upcoming war. But, then, when she also felt that the egg was in the Spine, she chose to buy it from Eragon and still move away with her family. Then, she still wanted to sell it to assure a better life for the Quimby, but ended using its power to learn magic and the egg hatched for itself after developing relationships with them. 

Second, she had to find a way to annihilate the Ra'zac to its extinction. And as there were only light and water that could stop them, she had to learn magic and it could be easier with a dragon egg with them. Except, Hallvar and Heimir convinced her to just get rid of those actually evil and maybe nurture the unborn ones. So they set up to find every black eggs.

Or ask the king about it.

When they stepped into the enchanted forest and encounter the elves, they learn magic and a way to find the black eggs and inspired the elves to move for the upcoming war by tracking the edulnari and making the king regret his doings. As such, the king and the ra'zac were out.

But it left the shade who could be Birgit's demise as he's also controlled by spirits. First, though, they get rid of the priests thanks to the urgal, so letting them live in Alagaesia, while Uru'baen is back to Ilirea. Second, they need Oromis and Glaedr to teach them how to subdue a shade or even get rid of wandering spirits in the Alagaesia.

Canon : 

Birgit friend of Ismira (mother of Katrina), wife of Sloan.

Quimby checks a cask turned by Morn.

Nolfavrell is 13, so 2 years apart of Eragon, 3 of Katrina and 4 of Roran

Headcanon :

Ismira and Sloan got Katrina at 17.

Birgit at 20, got Nolfavrell, with Quimby. 

Quimby is a Trader and 4 years older than his wife. And almost didn't want to be in relationship with her when she tricked him at 15 saying she was 16.

So when Nolfavrell was 13, Birgit was 33, Quimby 37. The twins, Hallvar and Heimir, are 4 yours younger, so 9.