How would chapters 25-26 change if they were told from the point of view of Mrs. Finney?

This is an interesting question because Mrs. Finney is a relatively minor character in Walk Two Moons. To look at these chapters from her point of view changes the experience for the reader quite a bit. 

First let's look at what happens in chapters 25 and 26. In chapter 25, Sal and Phoebe join the Finneys for dinner, which is a wildly chaotic experience in comparison to what they are used to. Dinner at the Finneys is not only more crowded than what Phoebe is used to, but also the food is less healthy than what Phoebe usually eats at home. Since Phoebe is struggling with the disappearance of her mother, she takes out some of her feelings on the dinner spread, much to her dinner mates' annoyance. After Sal and Phoebe leave the Finney household, Sal invites Phoebe to stay over at Sal's house. Then Phoebe returns home to find her father struggling with household chores and, of course, the absence of his wife. 

In chapter 26, Sal struggles with Phoebe's picky habits as a house guest and they return to the Finney house. Mr. and Mrs. Finney are playing in the leaves with a couple of their kids before they sneak off for an intimate moment. Ben tells Phoebe and Sal that Mary Lou is on a date. That night, Sal's father hears Phoebe crying by herself and Sal remembers when she was in Phoebe's position after her mother left. 

Right away we know that if those chapters are from Mrs. Finney's point of view, the reader is going to lose everything that occurs outside of the Finney household. This includes Phoebe's father's struggles at home, Sal's struggles with Phoebe as a house guest, and Phoebe's "birds of sadness." We're also going to miss out on Sal's feelings towards Ben and Sal's inside knowledge of Phoebe's mother's disappearance. 

Not a lot is known about Mrs. Finney, but we do know that she has a lot of children and a loving relationship with her husband. In chapter 25 she "straggles" in the door, is bombarded by her children, greets her husband, and serves fried chicken for dinner. From Mrs. Finney's point of view, this is most likely just another night until Phoebe voices her complaints about the unhealthiness of the dinner. Mrs. Finney doesn't know about Phoebe's missing mother, so from her point of view, Phoebe's attitude is probably a little annoying. Mrs. Finney seems a little exasperated to be lectured by Phoebe about cholesterol, but she finds Phoebe some muesli to eat and dinner continues in its usual hectic manner. In chapter 26 Mr. and Mrs. Finney are enjoying a Saturday with their children before they sneak away to the roof for some alone time. From Mrs. Finney's point of view, it's probably another normal Saturday. The reader gets a feeling that the Finneys' marriage is very loving and that they're able to sneak in some time for each other despite their chaotic lives. Mary lou is on a date, so maybe Mrs. Finney spends a little time thinking about that, but Mary lou isn't the oldest child, so even this is most likely not a novel event. 

So you can see that changing the point of view from Sal to Mrs. Finney changes the experience entirely. In Mrs. Finney's life, chapter 25 and 26 are full of completely normal events with the addition of Sal and Phoebe, but what are a couple of extra kids in an already full house? In contrast, from Sal's point of view, the events of her life are anything but normal. Phoebe's mother has disappeared, which brings up memories of her own mother's absence.