Perhaps she stumbles upon a secret vampire lair. Maybe she's exploring an old abandoned building and accidentally discovers a group of vampires. Or she could be researching the strange disappearances in town and her research leads her to the conclusion that vampires exist. In fanfiction, the possibilities are really endless as writers can be very creative in how they introduce this knowledge to the character.
In a lot of fanfiction worlds, Bella's sister is made to know about vampires to add more drama or a different perspective to the story. Maybe the sister discovers the truth about vampires in a moment of accidental encounter, like stumbling upon a vampire feeding or finding some evidence in Bella's room. This new knowledge can then drive the plot in various directions, such as trying to protect Bella from the vampires or getting involved with the vampire world herself.
One way is through a fight. If Victoria attacks Bella, it could leave Bella with physical and mental scars. Physically, she might be injured and have to recover, which could make her more resilient. Mentally, the trauma of the attack could make her more aware of the dangers around her, less trusting of strangers.
A possible plotline could be that Bella's new status as Emmett's sister makes her more confident in the vampire world. Since Emmett is a strong and outgoing vampire, he might teach Bella how to better use her vampire abilities if she were to become one in the story. This could lead to her getting involved in more dangerous situations but also growing as a character.
One possible storyline could be that Bella discovers her sisterhood with Jane and tries to use it to protect the Cullens from the Volturi. She might try to negotiate with Jane, using their family bond to sway her decisions regarding the Cullens.
Bella might become more assertive. Since Katherine is a strong - willed character, being her sister could force Bella to grow a backbone. She could start standing up for herself more and not be as meek as she is sometimes portrayed.
Maybe Bella has a warped sense of what's good for her sister. She could think that the vampire fanfiction world offers a form of escapism or a different perspective on life that she thinks her sister needs. It could also be related to a relationship she has with a character in the fanfiction. If there's a vampire character she admires or is in love with, she might want her sister to be part of that world so that she can better understand Bella's feelings towards that character.
Yes, in some vampire fanfictions, Bella is often depicted as the goddess of vampires. Writers like to give her such a powerful and exalted status to add more drama and mystery to the story.
One possible plot development could be that Bella discovers Wolverine's mutant abilities and they team up to fight against a common enemy. Maybe some evil mutants or a powerful organization that wants to use their powers for bad. Another could be that Bella has some special power herself which is only revealed through her connection with Wolverine as her brother.
One possible storyline could be that they discover a family secret together. Maybe something about their parents or their family heritage that has been hidden from them. This discovery could lead them on a journey to uncover more about their family's past.
Obviously, Paul and Bella's sister are the main characters. Paul might be portrayed as a charming and brave character. Bella's sister could be depicted as a kind - hearted and strong - willed person. There could also be secondary characters like Bella herself, who might play a role in either supporting or opposing their relationship.