In The Book of Unknown Americans, how is the scene where the Riveras sit down for a dinner of oatmeal a turning point for the family and for the book?

When the Rivera family sits down to dine on oatmeal, Maribel laughs. Both Arturo and Alma are shocked because they haven't heard her laugh since her injury.
Maribel has a traumatic brain injury. It affects every facet of her capability and ways of interacting with the world. It's also what drove the Rivera family to move to America; they hoped that she could benefit from special education programs there. While she's made improvements, she isn't back to her old self yet.
When Alma finds oatmeal at the store, she's skeptical about it. It reminds her of atole de grano that her mother made her in Mexico. However, it's like glue when she cooks it. It's a tasteless mush. She also makes it too early, so that it hardens as it sits out. Still, she encourages her family to taste it.
Maribel says the oatmeal feels weird. Arturo is also skeptical and asks Alma to repeat the name of it. He finds it funny and starts to laugh. Alma does too. She says the word is as formless and mushy as oatmeal itself.
Then Maribel laughs. It's the first time in a year that she's laughed, and it's a symbol of hope for the family.