One interesting Bella Swan's brother fanfiction could be about him having a secret power. Maybe he can communicate with animals, which gets him involved in some unexpected adventures in the Twilight world. This power could lead him to discover new secrets about the vampires and werewolves in the area.
One popular character paired with him is Jessica. In some fanfictions, they start as friends but then develop deeper feelings for each other. Their relationship is interesting because Jessica is so different from Bella, and it shows a different side of Bella's brother.
Percy's relationship with Jacob could also be a key part of the story. They might initially be at odds because Jacob is a werewolf and Percy is used to dealing with different kinds of 'creatures'. But over time, they could learn to work together, especially if they both have Bella's best interests at heart. Percy's interactions with Bella herself would be a mix of brotherly love and trying to keep her out of supernatural trouble.
For Harry, he might become more protective. Since Bella is his sister, he'd feel responsible for her safety. He could also become more in tune with the non - magical world, as he has a family connection there. For Bella, she could gain more confidence. With Harry's magical abilities at her disposal, she might not feel as vulnerable. Also, she might develop an interest in the wizarding world and learn some magic herself.
In some fanfictions, Bella's character might become more confident. As an immortal, she no longer has the same fears of mortality. For example, she could be more fearless in facing dangerous situations.
One way the storyline could be is that Percy uses his demigod powers to protect Bella from the various threats in Forks. For example, he could sense the presence of vampires and werewolves before Bella does and keep her out of harm's way. He might also introduce her to the world of Greek mythology, which would be a huge contrast to the world of Twilight she already knows.
She might be portrayed as a villain by showing her being manipulative. For example, she could use her relationship with other characters to get what she wants, like using Edward's love for her to control his actions or decisions.
Well, in some fanfictions, she might be depicted as the devil through her actions. Maybe she's shown as being manipulative, using her charm to get what she wants from others, much like how the devil is often portrayed as a tempter in traditional stories.
She might become more assertive. As the daughter of Ares, she would inherit some of his war - like traits. So, instead of being the somewhat passive Bella in the original story, she could be more proactive in making decisions and taking action. For example, she may no longer wait for others to protect her but instead take the lead in dangerous situations.