If Bella was Stefan and Damon's sister, her relationship with Elena would be very interesting. Elena might see her as a sister - in - law of sorts, but also as a potential rival for the Salvatore brothers' attention. With Caroline, they could become good friends as Bella might be more understanding of Caroline's transformation into a vampire. And with Klaus, he might see her as a new target or an interesting addition to his games, since she's related to the Salvatores.
When it comes to her relationship with Tyler, they could either be friends or enemies. Tyler, as a werewolf, has his own issues with the vampires in Mystic Falls. If Bella tries to promote peace between the vampires and werewolves, Tyler might be open to it, but if she sides too much with her vampire brothers, he could see her as an enemy. Also, her relationship with Matt would be a bit of an outsider - looking - in kind of thing. Matt, being a human, might be curious about Bella and how she fits into the supernatural world of Mystic Falls while still trying to protect his own humanity.
Her relationship with Bonnie would be complex. Bonnie, as a witch, might be initially suspicious of Bella because of her connection to the vampire brothers. But as time goes on, they could form an alliance, especially if there are threats to Mystic Falls. Bella's relationship with Alaric could be one of respect, as he's a vampire hunter turned vampire himself. He might see her as a different kind of vampire compared to Stefan and Damon, and she could learn a lot from him about the history of vampires and how to control her new nature.
Well, one possibility is that Bella's presence completely changes the dynamic between Stefan and Damon. Maybe she's the mediator in their constant feuds. Since she's their sister, she might have a special power that could either help or hinder them in their vampire world. For example, she could have a unique ability to sense danger that the brothers don't have, which would make her an important asset in their battles against other supernatural beings.
One idea could be that Bella is a sweet - natured girl who is caught in the middle of Damon's wild ways and Stefan's more reserved nature. She might look up to Stefan as a role model for being good, but at the same time be intrigued by Damon's sense of adventure. In the fanfiction, she could end up having to choose sides when a big crisis hits their family, like an attack from a powerful vampire enemy.
One common theme is the struggle for acceptance. Since Bella is the mate of both Damon and Stefan, there's often a plotline about how the vampire community or their friends accept this unusual relationship. For example, they might face judgment from other vampires who don't understand this triad.
It changes drastically. Usually, the trust is broken. Bella may become very guarded and distant towards Damon.
Bella might be very empathetic. Since she's in a family full of complex emotions, she can sense when Damon is feeling particularly broody or when Stefan is stressed. So she tries to comfort them in her own way, like making them a special meal or just sitting and listening to their problems.
One great fanfiction could be 'Bella's Bond with the Brothers'. It explores Bella's relationship with Damon and Stefan in a really interesting way, showing how she fits into their world of vampires and the unique dynamics that develop among them.
Caroline could be in the story. She might be Bella's friend or a rival. If she's a friend, she could help Bella navigate the supernatural world. If a rival, it could be because of her relationship with Damon or Stefan in some way. Also, Klaus could be a character. He might see Bella as a means to get to Damon, or he could be interested in her unique abilities if she has any.
There could be Bella's mother. She would be an important character as she was part of the relationship that led to Bella's birth. Maybe she has some secrets or reasons for keeping Damon away from Bella all these years.
Her relationship with Bella would likely become more complex. As a vampire, she would have different needs and instincts. She might be more protective of Bella in a way that Bella doesn't understand at first. For example, she could sense danger towards Bella that a human wouldn't notice. But at the same time, Bella might be a bit afraid of her sister's new vampire nature, which could create some tension between them.
Yes, in many fanfictions, Bella is often written as Stefan's mate. Writers like to create crossovers between different fictional universes and explore new relationships. It adds an interesting twist to the characters' stories.