Since I have no knowledge about the 'Sarah Short Story' in question, it's impossible to say who the author is. It could be an unknown writer, or it could be a story written by a beginner as a part of a writing exercise. There are so many possibilities.
I'm not sure if she is popular in Fantastic Fiction. Popularity can vary depending on different factors such as the number of her books available, the readership, and the reviews.
I'm not sure if Sarah Castille is widely known specifically in Fantastic Fiction. It could depend on how much exposure her works in this genre have had, as well as how popular they are among the readers of Fantastic Fiction.
The relationship could also develop in a more platonic way. They could be like sisters, supporting each other during doctor's appointments and prenatal classes. They might have arguments along the way, but always come back to support each other because they know how tough pregnancy can be.
One possible fanfiction could be where Addison and Wyatt go on a secret adventure outside of Seabrook. They discover a hidden place filled with old zombie artifacts. Wyatt is hesitant at first, but Addison's enthusiasm encourages him to explore. Along the way, they face some challenges that test their friendship and also learn more about each other's pasts.