Well, it could be due to a number of reasons. Maybe in the fanfiction, Bella has some unique abilities or qualities that Elena lacks. And this makes Elena feel envious. Also, if the story portrays Bella as getting more attention from other characters, especially those that Elena values or is interested in, that would surely make Elena jealous.
Elena might show her jealousy by being overly critical of Bella. For example, she could constantly find fault in Bella's actions or decisions in the fanfiction.
In this fanfiction, Bella is likely the main focus. Because the whole premise is about Elena being jealous of Bella, which implies that Bella is at the center of whatever relationship or situation is going on with Damon. Elena's jealousy is directed towards Bella, so the story would probably revolve around Bella and how she deals with Elena's jealousy and her relationship with Damon.
One possible scenario is when Bella is more successful in achieving a common goal in the fanfiction world. For instance, if they are both after a magical artifact, and Bella gets it first. Another scenario could be that Bella has a closer relationship with a mentor figure, which Elena covets. It might also be that Bella has a more glamorous appearance and gets more admiration from other characters, making Elena jealous.
By showing them having heart - to - heart conversations. For instance, late at night when they can't sleep, they talk about their dreams, fears and hopes. This can deepen their understanding of each other.
One aspect could be that in such fanfiction, jealousy often adds a new layer of drama to the relationship between Edward and Bella. For example, if a new character is introduced, and either Edward or Bella shows jealousy, it can lead to interesting conflicts. It might also explore their insecurities within the relationship more deeply.
Maybe Elena just doesn't like the way Bella is portrayed in fanfiction. It could be that she has a different view of the original Bella character and the fan - made versions deviate too much from it.
One popular trope is the over -protective jealousy. Edward, being over -protective of Bella, gets jealous easily when he thinks she might be in danger from other male characters. It shows his deep -seated fear of losing her. Also, there's the 'green -eyed monster' trope where Bella's human insecurities surface as jealousy when she sees Edward's supernatural allure attracting others.
In fanfiction, Bella and Elena being cousins can open up various storylines. Maybe they could have a shared family secret. For example, they might discover they are part of a long - lost magical family line. Their relationship could be full of sisterly love and rivalry at the same time. They could compete for the attention of a certain character in the fanfic world, or they could team up to face a common enemy.
One possible scenario in this fanfiction is that they both have different hobbies. Bella might be into reading and Elena into painting. At first, they might be a bit annoyed by each other's hobby - related messes. But over time, they start to appreciate and even participate in each other's hobbies. For example, Elena might start reading some of Bella's favorite books and Bella could try her hand at painting, leading to a deeper bond between them.