When Nettie tells Samuel and Corinne the entire story about Celie, what shocks Samuel the most?
When Nettie tells Samuel and Corinne the entire story about Celie, the thing that shocks Samuel the most is that Celie's children, Olivia and Adam, are the result of Celie's rape at her own father's hands.
In the beginning of the story, the "Dear God" letters tell us that Celie's father raped her when he couldn't get Celie's mother to have sex with him. Immediately after Olivia and Adam were born, Celie's father took the babies away from Celie. Samuel and Corinne eventually adopted the children, but as time went by, Corinne began to suspect that the children were the product of an affair between Nettie and Samuel.
Nettie tries to reason with Corinne, but to no avail. She points out to Corrine that Corinne has seen the lack of stretch marks on her stomach. Nettie argues that stretch marks are not something that can be wiped out; only women who have ever experienced pregnancies get stretch marks, and she doesn't have any. Nettie also tries to remind Corinne of the time she met Celie in town. She believes that, if Corinne can just remember, she will see how the children actually resemble Celie.
To jolt her memory, Nettie pulls out a quilt that Corinne had made. She asks Corinne whether she remembers buying the cloth for the quilt. With tears in her eyes, Corinne eventually admits that she remembers. She confesses that Olivia had looked so much like Celie that she was afraid. Her fear was that Celie would want her children back. Even though she suspected then that Celie might have been the real mother of the children, Corinne didn't want to face the truth. Instead, she decided to forget about Celie.
Corinne eventually dies after her admission, but not before she tells Samuel and Nettie that she now believes the truth about the children's maternal heritage.