What happens when Rose watches the motion picture, Man in the Iron Mask, in the novel, We Are All Completely Beside Ourselves?

When Rose watches Man in the Iron Mask she has a panic attack. The story is about two twins, one destined to rule and the other imprisoned. The twins switch places so the one that should rule was instead imprisoned. Rose panics because she feels guilty over what happened to Fern. In her mind, she was the one that should have been imprisoned, but she got swapped out and got to live the good life while Fern was locked away in a cage.