Well, perhaps the story could revolve around the sister's past before becoming a vampire. She might have had a human life full of secrets and unfinished business. In the fanfiction, she could be trying to resolve those while dealing with her new vampiric nature and her relationship with Bella. For example, she might have had a human love interest and now she has to figure out how that fits in with her new existence as a vampire sister to Bella.
One possible plotline could be the exploration of the relationship between Bella and her vampire sister. Maybe they have to face some challenges together, like dealing with other vampire clans who see them as a threat. Another could be their journey in discovering their unique powers as sisters in the vampire world.
One possible plotline could be the power dynamics within their family. Since Aro is a powerful vampire in the Twilight universe, Bella being his sister might lead to her having unique powers or a special status. Another could be the love triangle situation. If Bella is Aro's sister, how would that affect her relationships with Edward or Jacob? It could add a whole new layer of complexity to the story.
In some 'Bella Swan as a Vampire' fanfiction, a great plotline is her exploration of her new vampire senses. She could find that her sense of smell leads her to discover things about the past of the places she visits. And in terms of relationships, her bond with Edward could change in a really interesting way. Since they are both vampires now, they might have different power dynamics and new challenges in their relationship. For instance, they could compete in terms of their vampire skills, which could either strengthen or strain their relationship.
One possible plotline could be Bella adjusting to her new vampire abilities. For example, she might struggle with her increased strength at first and accidentally break things. Another could be her relationship with Edward changing as they are now both vampires. Maybe they have newfound understanding and experiences to share.
One possible plotline could be the discovery of their relation and how it shakes up the vampire world. For example, Aro might be more protective of Bella than he is of other vampires. Another could be Bella having to navigate the Volturi politics because of her newfound family tie.
Another plotline could revolve around her relationship with Bella. For example, she might be jealous of Bella's relationship with Edward at first but then come to understand and support it. She could also have her own love interest within the Twilight universe, perhaps a new vampire or werewolf character that was introduced just for the fanfiction.
One interesting plotline could be Bella's struggle to control her new vampire instincts while still maintaining her relationships with her human friends and family. For example, she might have a hard time being around them without revealing her new nature.
A third plotline might be that Bella and Edward face a new kind of threat to their vampire family. Maybe a group of powerful and evil vampires from a far - off land come to Forks, and they have to use all their skills and the help of their friends to protect their family and way of life.
One interesting plotline could be Bella discovers Edward's vampire nature in a more dramatic way than in the original. For example, she might witness him using his super speed to save her from an immediate danger like a speeding car. Another could be that Edward is initially more hesitant to be around Bella because he fears his vampire instincts more strongly, creating more tension in their relationship.
Well, a common plotline is Bella's discovery of new vampire senses. In fanfiction, it could be that she discovers she has a unique sense that other vampires don't have, like being able to sense the emotions of humans from a great distance. Also, there could be a plot where Bella has to hide her new vampire self from her human family and friends, which creates a lot of tense and emotional moments as she tries to keep her double life a secret.