What big part of the story was revealed in chapter 27?

In Chapter 27, Merci continues to hear the bass from the radio, then hears sirens. The back of the car is smashed in. A nurse says they were fortunate and that “God was looking out for you” (264), but Merci wonders why God was not looking out for Lolo. She later learns that Abuela and Lolo had gone to the store to get more Halloween candy, and that Lolo wandered away. That night, Merci wishes to be near Roli but goes to the window to look at Lolo and Abuela's house. Enrique is sleeping there. Roli wakes and Merci asks what is causing Lolo's behavior. Roli considers his answer for awhile, then begins to explain. He got a life-sized rubber model of a human brain for Christmas one year, and he uses that to demonstrate what Alzheimer's is doing to Lolo's brain. He says Lolo's brain is shrinking and some parts no longer work well. He says Lolo will only get worse. Merci realizes this diagnosis was made a long time ago and that the adults chose to keep it a secret. She picks up the brain model, taking it apart and putting it back together as she learns to say the words that make up the parts of the brain. When she has mastered that, she begins to ask Roli questions and he “answers every single one” (268).