but also serves as a mother figure to the fugitive Davy and an equally fugitive girl with little connection to the family.

Reuben and Swede, particularly Reuben, are a bit distrustful of Roxanna at first, but Roxanna is a quiet, steady presence in their lives and soon they are won over. On the first night they meet, she helps Jeremiah get Reuben through an asthma attack, and it seems as if she is part of their family from that moment on, despite the children's misgivings. 

The authorities eventually descend on Roxana's home (because of the Lands, of course); and "from the first, Roxanna offered them fragrant breads or pastries and otherwise kept her silence." She is a wise and comforting woman, but she is also tough and durable for the hard things that will come. When she leaves everything behind to leave with her new family, Reuben is impressed by how easily Roxanna makes the transition from her old life to her new one.

It then occurred to me that this leaving--which to me ached with failure and despair--was for her the commencement of a gallant endeavor. Who isn't scared by as whole a redirection as that on which she now embarked? Adhering to us must've seemed a risk demanding the deepest reserves of joy and strength....Glad though I was to have her along, it would be years before my gratitude approached anything like proportion.

Roxanna is a woman who has been hurt but still knows how to love, and that makes her a perfect match for the Land family. When Jeremiah is gone not very much later, Roxanna is deeply entrenched in the family and is able to be the connection the children need to each other and to their father. 

You are only allowed one question per eNotes post, so I kept your first one. In Peace Like a River, Roxanna is the only real, normal adult female. Swede and Reuben, and even Davy, are relatively well adjusted children despite not having a mother figure in their lives. In fact,  they are rather exceptional children, undoubtedly due to their exceptional father. When Roxanna enters their lives (or they enter hers), there is a new balance in the family. She offers stability and "normalcy" to this otherwise rather quirky family; yet she is also quirky and out of the ordinary. It takes a strong woman to be relaxed and flexible enough to adapt to the Land family dynamic, and Roxanna is just that. The courtship between Jeremiah and Roxanna is sweet and pure, and their relationship (then marriage) provides a calmness and stability which this family is craving after so much instability and outrageousness.

Reuben Land is the young, and rather precocious, boy who serves as the narrator for Peace Like a River, by Leif Enger. Miracles have consistently happened in this family since the day Reuben was born; in fact, Reuben is only alive because his father's prayers were answered.

His family--father Jeremiah and sister Swede--are on a quest to find his older brother, Davy, who is on the run from the law. The family is traveling in an Airstream trailer in the bitter cold, looking for a place to buy propane and gasoline, when they stop at an isolated farmhouse with two pumps out front. This is where they meet Roxanna.

Though Reuben is not particularly unkind toward Roxanna, it is clear from his descriptions of her that he has some doubts and is reserving judgment. He is wryly taken aback when Swede responds immediately to Roxanna, and then he says (to the readers):

It may surprise you, after [learning about] the goats in the bathroom, that Roxanna Cawley set a pleasant and even cultivated table.... I suppose it was a meal intended to impress, though you don't think of a woman like Roxanna worrying about how her hospitality comes off; she hadn't seemed at all ashamed about the goats.

While this kind of commentary implies that he expected her to be more rustic and uncouth, he also reveals his grudging respect for her. 

As Reuben learns more about Roxanna, he realizes that he and Swede have much in common  with her. Things change drastically that night when Reuben has a breathing episode. Roxanna thumps his back as Jeremiah instructs her and then sits next to Reuben on the bed afterward; before she left, "she bent and put her cheek to mine. Her hair was in a single thick braid and moist coils of it had come free--they clung to my face as she pulled away."

From this point on, Reuben loves Roxanna. When she and Jeremiah fall in love, Reuben is happy for them and for the addition of Roxanna to their lives. At one point, however, Reuben writes:

At this a dread realization occurred: Since arriving at this house, we'd had no miracles whatever.

This is a horrifying realization for Reuben, and he suffers for a time from this knowledge as well as guilt over his discovery of Davy and then jealousy because Roxanna opens her heart to others. In the end, though, it is at Roxanna's house that Reuben experiences another transformational miracle: his father dies in his place. Roxanna will be part of Reuben's life forever, now, and he will love her like a true mother.