One common plotline could be the sisters dealing with their different personalities. Rosalie is often seen as more confident and assertive, while Bella is more introverted. So, they might have conflicts and then reconciliations as they learn to understand each other better. For example, Rosalie might try to get Bella to be more outgoing, which Bella initially resists.
Another plotline could involve them facing some external threat together. Since they are sisters in the fanfiction, they would have to rely on each other for support. Maybe there's a new vampire coven in town that is causing trouble, and they work together to protect their family and friends. They would combine their unique skills - Rosalie's physical strength and Bella's ability to empathize with others - to defeat the threat.
One common plotline is Rosalie and Bella bonding over their shared experiences in the vampire world. For example, they might team up to face a new threat to their family or coven. Another plot could be Rosalie teaching Bella some of the more refined vampire skills, like how to use her enhanced senses in a more sophisticated way. There are also fanfictions where they explore their sister - like relationship by having heart - to - heart conversations about their past traumas, which helps them grow closer as sisters in the story.
One interesting plotline could be them bonding over a shared interest, like fashion. Rosalie's love for all things glamorous could rub off on Bella, and they could go on shopping sprees together. Another could be them facing a common enemy in the vampire world and having to rely on each other's unique strengths. For example, Bella's self - sacrifice nature and Rosalie's physical strength.
A plotline could also involve the teacher being a supernatural being as well. For example, the teacher could be a wizard who has a connection to the vampire world. In this case, the teacher might be training Bella and Rosalie in some special skills or trying to use them for their own purposes. It would create a very interesting dynamic between the three characters.
There is a fanfiction titled 'Rosalie and Bella: Sisters in Secret'. In this story, they have to keep their sisterly relationship a secret from the rest of the Cullen family for some unexpected reasons. It's full of mystery and family drama.
Another plot could involve Rosalie teaching Bella some of her more refined vampire skills, like how to move with extreme grace and elegance. This could lead to a deeper bond between them as Bella starts to understand Rosalie better. In the fanfiction, this newfound closeness might also stir some unexpected emotions among the other Cullens.
One interesting plotline could be how they navigate their relationship in a world full of vampires and werewolves. For example, they might face challenges from the Cullen family who have different views on their relationship. Another could be their journey to understand and accept their unique bond as mates, which might involve them going on adventures together to discover more about their vampire nature.
In some fanfics, male Bella might be a vampire hunter who initially targets Rosalie but then realizes she's not as bad as he thought. This creates a really interesting 'enemies to lovers' type of plotline.
In twilight Rosalie and Bella fanfiction, a great plotline could be Rosalie helping Bella adjust to her new vampire life in a more motherly way. Since Rosalie has been a vampire for a long time, she has a lot of knowledge and experience. Maybe she could help Bella with controlling her new powers and dealing with the emotional changes. There could also be a plot where they go on a journey together, away from the Cullen family for a while, to discover more about themselves and their place in the vampire world. This journey could bring them closer and make them see each other in a whole new light.
Their personalities interact in various ways. Rosalie, being proud and protective, might initially be overbearing towards Bella. However, as time passes, she realizes that Bella has her own strength. Bella, being gentle and curious, might be intrigued by Rosalie's confidence. She may start to imitate some of Rosalie's behavior in a positive way. Also, their different approaches to relationships, with Rosalie being more guarded and Bella more open - hearted, can lead to interesting conversations and developments in the fanfiction.
Perhaps they embark on a journey to discover more about their own origins. This could involve traveling to different places, meeting old vampires who might know something about their past. Along the way, they learn more about each other and grow closer as sisters.