The plot might revolve around Bella's sister being framed for a crime she didn't commit. In the Twilight - like setting, perhaps she's accused of murdering a vampire, which is a big deal. Bella then uses her connections with the Cullens to try and prove her sister's innocence. They have to go through a lot of detective work, like looking for alibis and finding the real culprit who might be another vampire with a vendetta.
Well, one possible plot could be that Bella's sister gets involved in some illegal activities in the vampire - human world of Twilight. Maybe she's stealing vampire artifacts to sell on the black market. And Bella has to decide whether to turn her sister in or help her cover it up to protect their family's reputation.
In a 'Bella Swan Criminal Sister' fanfiction, the sister could be a criminal mastermind. She could be secretly working with the Volturi's enemies. Bella discovers this and is torn between her loyalty to her sister and her loyalty to the Cullens who are in a tenuous peace with the Volturi. She tries to persuade her sister to stop, but the sister is too far gone in her criminal plans and Bella has to find a way to stop her without causing a full - scale vampire war.
A powerful werewolf could be the main antagonist. In the fanfiction world, werewolves and vampires often have a tense relationship. If Bella's sister does something that angers the werewolves, like stealing from their territory or harming one of them accidentally while doing her criminal deeds, the werewolf pack leader could become the main antagonist, vowing revenge on the sister and by extension, Bella and the Cullens.
Maybe she teams up with other characters who are also on the wrong side of the law. Together, they plan heists or other illegal activities. This would introduce new dynamics and relationships not seen in the original story.
One possible plot could be that Bella's discovery of being Aro's twin sister completely changes the dynamic in the Twilight world. Maybe she has some of Aro's powers, like the ability to read minds to an extent. This could lead to her being a target for other vampires who fear her new - found powers and connection to the Volturi.
In a 'Bella Swan sister witch twins mate' fanfiction, the plot could start with the sister witch having a premonition about the twins. She realizes that their mating is crucial for some greater purpose in the supernatural world. Bella is initially skeptical but soon gets involved. As they get closer to the twins, they find out that there are ancient prophecies surrounding them. The group has to fight against an evil coven or a group of rogue werewolves who want to prevent the twins from mating. The sister witch uses her powers to fight, Bella uses her knowledge of the Twilight world, and the twins have their own unique abilities that come into play. Throughout the story, the relationships between all the characters evolve, with love, friendship, and loyalty being tested.
One possible plot could be Bella's struggle to fit into the wizarding world of Malfoy Manor. She might be different from the traditional Malfoy family values, being more empathetic like in her 'Twilight' self. She could face inner turmoil as she tries to balance her newfound family loyalty to the Malfoys with her own sense of right and wrong. For example, when Draco is given dark tasks by Voldemort, Bella might try to persuade him not to go through with it, creating a rift in their relationship as Draco is initially more inclined to follow his family's dark - side - leaning traditions.
The plot could revolve around Bella Swan's sister being in a dangerous situation. For example, she could be kidnapped by a group of rogues who are after Bella. In the process of trying to escape, she finds herself in a sacred place of the werewolves which somehow transforms her. As a wolf, she uses her new abilities to fight back and protect herself. She then has to adapt to her new life. She may struggle with the idea of being a wolf as she was previously just an ordinary girl, but over time, she learns to embrace her new identity and uses it to help protect Bella and her family from future threats.
One possible plot could be that Kol's twin sister, being a vampire in the Twilight universe as Bella Swan. She might be intrigued by Bella's relationship with the Cullens. Maybe she tries to interfere in their relationship out of jealousy or a desire to understand human - vampire love better. Since she has her own vampire powers from the Mikaelson lineage, it could create some interesting conflicts with the vampires in Forks.
Maybe Bella Swan discovers Cortez's work and it changes her perspective on something. For example, if Cortez is an artist, his art could inspire Bella in her own life in Forks.
Well, it could be that Bella is secretly an assassin all along in Forks. She's using her relationship with Edward as a cover while she's on a mission to take out some supernatural threats in the area.
One possible plot could be that Bella Swan, in this fanfiction, becomes a huge admirer of the Gilbert family. Maybe she is intrigued by their history in the town. She could start to study their family tree, trying to understand the secrets they hold. This could lead her to befriend members of the Gilbert family and get involved in their daily lives, which might be full of both normal family drama and some supernatural elements as in the original 'Twilight' and 'The Vampire Diaries' universes where the Gilberts are from.