One common theme is the idea of forbidden love. Since the man is older, there are often societal or supernatural barriers that make their relationship seem wrong or difficult. For example, in some fanfictions, the older man might be part of a different vampire clan with strict rules against relationships with humans like Bella.
One author to look out for is RoseThorne. Their stories often feature the 'Bella meets older man Edward' trope in really interesting ways. They are good at creating tension and building a believable relationship between the two characters.
One popular 'Bella meets older man Edward' Twilight fanfiction might be 'Eternal Bond'. In it, their relationship develops in a more mature and complex way compared to the original. It delves deeper into the age difference and how society in the Twilight world views it.
One popular 'Bella meets older man' Twilight fanfiction is 'Unexpected Encounters'. In this story, Bella crosses paths with an older vampire who has a mysterious past. It adds an interesting twist to the Twilight universe.
One possibility is that their relationship could face initial resistance. In many Twilight fanfictions, Bella's friends or family might be concerned about the age difference. They could try to keep them apart, creating drama and tension in the story.
Their relationship might develop slowly at first. Since Edward is older, he could be more cautious. He might take his time getting to know Bella on a deeper level before fully committing to a relationship.
It might be that Caius is on a mission outside of Volterra. He crosses paths with Bella's sister in a small town. She is doing some mundane task like shopping, and he is there to hunt (not for her, of course). But her scent catches his attention, and he approaches her. She is shocked to see such a pale and intimidating figure, and this is their first encounter.
One popular 'Bella's older sister' Twilight fanfiction might be 'The Forgotten Sister'. It often delves into how the dynamic of the Cullen family changes with an additional sister figure. Another could be 'Sister of Bella: A New Twilight'. In this, the older sister character might bring new plot twists, like having different relationships with the werewolves or vampires compared to Bella.
When writing a 'Bella's older sister' Twilight fanfiction, it's important to study the original Twilight series. Know the rules of the vampire and werewolf lore. Then, start building your character. Give her a backstory, like where she was before coming to Forks. You can make her have a different relationship with the Cullen family. For instance, she could be friends with one of the Cullen siblings before Bella even arrives. Develop the plot around how she affects the events that Bella goes through in the original story. You could also add new characters that interact specifically with the older sister.
She could meet soldiers coming back from World War I. These soldiers would have their own stories of war and survival, and Bella might be drawn into their tales.