In his real life, Edgar Allan Poe had many devastating experiences with death that tormented him the rest of his rather short life. He was so tormented by the deaths of his family members that, in Avi's novel The Man Who Was Poe, the character Edmund very correctly realizes that Poe has become afraid of living, as Edmund asserts in the final chapter, just before he and Poe part ways:
You're always talking about death, ... but it's living you're frightened of. (p. 198)
All throughout the novel, Poe speaks of his fears such as the fears of being haunted by a ghost, who was really Edmund's mother in the cemetery looking for her daughter, and tormented by demons, who were really nothing more than people at a party who criticized Poe's work. Furthermore, Poe explains that the reason why he writes about evil in the world is because "writers write about what they know best. And ... what some writers know best is what they fear" (p. 117). These fears of his developed partially in response to the experiences of death he suffered, first the death of his mother when he was only three years old then the death of his young wife, whom he called Sis, just as Edmund calls his sister Sis in Avi's story.
It's because Poe's own Sis died in his own real-life story that Poe can't picture Edmund's Sis as doing anything but dying in Poe's version of Edmund's story. Edmund realizes Poe's inability to see Edmund's Sis live stems from Poe's fear of the continuation of life, since the continuation of life brings nothing but more fears, more sorrows, and even more death.
While Edmund is correct in his assessment of Poe's actions, Poe retorts by explaining that Edmund's Sis would have lived eternally if her death had been captured on the pages of his story. Poe's response shows us that Poe has come to cope with his griefs through his writing, because he sees his writing as being able to create that which can live eternally.