Well, a plot could revolve around Edward and Bella's step - parent relationship. Maybe their new parent has some secrets that are affecting the family. Edward and Bella team up to figure out what's going on. Also, they could be competing for their parents' attention at first, but then realize that they can support each other instead. And there could be a sub - plot where they both get involved in a community project together and through that, their step - sibling bond becomes even stronger.
In general, it can be appropriate as long as it doesn't cross any lines. Fanfiction often takes existing characters and puts them in new situations. If the 'Edward and Bella step siblings fanfiction' is about their new family dynamic, like how they deal with being step - siblings, it's okay. But if it has any inappropriate undertones related to their new relationship, then it's not suitable.
One possible plotline could be Bella initially resenting Edward as her step - dad but then gradually coming to understand him better and they form a unique bond. Another could be Edward struggling with his new role and having to deal with his own feelings for Bella in a more platonic way than in the original story.
Well, a plot could involve Bella and Jasper having a secret language or a special bond that only they understand. This could lead to some really interesting scenarios where they use this bond to outwit their opponents. Also, perhaps they discover a long - lost relative who is a key to unlocking some hidden power within them.
One idea could be that they discover a long - lost family secret together. Maybe there's a hidden inheritance or a special power that runs in the family that only they are destined to uncover.
Well, in this kind of fanfiction, a plot twist could be that an outside force tries to drive them apart by making them believe false things about each other. Maybe they are both in love with the same person, which creates a huge rift between them. Or there could be a situation where one of them sacrifices themselves for the other in a way that no one expected, completely changing the dynamic of their relationship.
One idea could be that they go to a secluded beach and discover a hidden cove with magical properties. Another could be a trip to a foreign city where they encounter a vampire from an ancient lineage. And they might also go on a mountain vacation and face some dangerous supernatural creatures there.
One idea could be that Edward is a criminal on the run and Bella, a journalist, is trying to expose his crimes but ends up falling in love with him. Another could be Edward as a criminal mastermind who recruits Bella into his world of crime. Maybe Edward is wrongly accused of a crime and Bella helps him prove his innocence while they face many dangerous situations together.
One interesting plot idea could be a love triangle where both Edward and Emmett are vying for Bella's attention in a more complex way than in the original story. Maybe there are external forces at play that make Bella question her feelings for both of them.
Well, perhaps in this fanfiction, the shower is a place where they find a magical or symbolic element. It could be that the water has some sort of special property that helps Edward control his thirst for Bella's blood in a new way. Or, the plot could revolve around them getting into an argument in the shower, which then leads to a passionate reconciliation as they realize how much they mean to each other and how they need to communicate better.
Perhaps in the fanfiction, Bella is also in prison for some unjust reason. Edward, being a vampire, has to use his special abilities secretly to protect her from the other inmates. He could be using his mind - reading skills to anticipate threats.