Anna Witting, Jacob's oldest child, is the storyteller of the book because the entire story is told from her perspective. But it isn't just her narrator status that makes her a storyteller. Anna also tells stories to her little brother Caleb, like one about what happened when he was born:
"And Mama handed me to you in the yellow blanket and said…" He waited for me to finish the story. "And said…?"
I sighed. "And Mama said, 'Isn't he beautiful, Anna?'"
"And I was," Caleb finished. (1.15-17)
Anna tells Caleb all the happy stories of what their family used to be like—stories about their mother and how their parents used to sing together. She tries to keep these memories, which Caleb doesn't have at all since he was just a baby when their mother died, alive for him. In this way, we might even think of Anna as the family historian, making sure the past is remembered and carried forward.
That said, there are certain stories that Anna keeps from Caleb. He's blissfully unaware of how awful it was for Anna after their mother died because she protects him from these memories:
I looked at the long dirt road that crawled across the plains, remembering the morning that Mama had died, cruel and sunny. They had come for her in a wagon and taken her away to be buried. And then the cousins and aunts and uncles had come and tried to fill up the house. But they couldn't. (1.20)
Anna remembers what her family was once like—and what it became after their mother died. This is why she's so invested in things working out with Sarah: She wants Sarah to marry Jacob so her family will look the way that it did when Anna was little and she had both a mother and father. She remembers what it's like to have two parents, and she wants this experience back.
Because Caleb has grown up without a mother, he's relied on Anna to take care of him—and she's definitely risen to the task. She always thinks of her brother first and tries to be strong for him, even when she's scared or sad:
A few raindrops came, gentle at first, then stronger and louder, so that Caleb and I covered our ears and stared at each other without speaking. Caleb looked frightened and I tried to smile at him. (8.33)
Anna is a kid, too, but she tries to take on a more adult role for Caleb so he has someone to turn to when he's afraid or sad. She's there for her brother whenever he needs her, acting like a little mom in her own right.
As the book ends, though, and Sarah's settling into life with the Wittings, it seems like Anna is finally getting a bit of mothering for herself, too. Will she and Caleb always be close? Probably—they've been through a lot together—but it also seems like Anna will get to be off mom duty a bit, finding some room to be a kid herself once more.