What are Isabel's most fearful moments?

This answer is going to depend on the reader. Readers will all see Isabel in fear throughout the story, but picking the one or two most fearful moments will change slightly from reader to reader; therefore, I am going to list many instances when I feel that Isabel was experiencing more than her standard amount of fear and worry.
Isabel realizes that she and Ruth are not being set free—instead, they are being resold into slavery to the Locktons.
Isabel realizes that Madam Lockton knows that she can further control Isabel by taking advantage of Ruth. One sick way that Madam Lockton does this is by turning Ruth into her own personal living human doll child.
Isabel watches Madam Lockton threaten to hit Ruth. This is what causes Isabel to throw herself down and take the beating instead of Ruth.
Isabel gets branded . . . on the face.
Isabel is told that Madam Lockton sold Ruth.
Isabel rescues Lady Seymour from the fire. This had to be a scary and harrowing experience.
Isabel fakes her papers to become free.
Isabel rescues Curzon from the prison.
Isabel rows across the river knowing that she and Curzon could be captured at any moment.