One theme that might show up is protection. Eric could be very protective of Bella, or vice versa. It could also involve themes of acceptance, especially if their relationship is different from what others expect. For example, if they come from a different background or have unique abilities. There could also be themes related to growing up together and the shared experiences that shape their characters.
The theme of identity can be quite prominent. Bella being Eric's sister might make her question her own identity in relation to him and their family. Additionally, themes of friendship can be intertwined as they interact with other characters. Their relationship as siblings can also influence how they form and maintain friendships with others in the fanfiction world.
One common theme is the exploration of a new relationship. Since Bella and Eric are not mates in the original, it's all about how they come to be mates in the fanfiction.
A third theme could be self - discovery. Through their relationship, whether it's a new friendship or something more complex, both Bella and Eric can learn more about themselves. In some fanfictions, Bella might discover a different side of her personality through her interactions with Eric, and Eric might gain more confidence in himself as a result of being around Bella.
Often, you'll find the theme of power and control. Eric is a powerful vampire in his own right, and Bella has her own kind of power, whether it's her connection to the Twilight vampires or her own inner strength. Their relationship often explores how they balance power within the story. Also, the theme of loyalty comes up a lot. They both have loyalties to different groups or individuals, and how they navigate those loyalties when they are together is a big part of the fanfictions.
Another theme could be growth. As the story progresses, both Bella and Brian might grow as individuals, learning from their experiences together. For example, if they face a difficult situation like a natural disaster, they come out stronger on the other side. In many of these fanfictions, friendship is also a theme. They make new friends while going through different adventures, and these friendships also contribute to their growth.
Love and protection are also prevalent themes. Peter may be very protective of Bella, and Bella might have a deep love for her brother. This can be seen in the way they interact and the sacrifices they are willing to make for each other in the fanfiction stories.
One common theme is family loyalty. Since they are sisters, the stories often explore how they stay loyal to each other no matter what. Another theme is the discovery of self. They might learn more about themselves through their relationship with each other. Also, there can be a theme of acceptance, as they accept each other's flaws and strengths within the sisterly bond.
One common theme is family dynamics. Since it's about Bella's sister, it often explores how the sister fits into the family structure, especially with Bella being so central in the original Twilight story. Another theme is love triangles. Just like in the original, but now with the sister involved in some way.
The themes often revolve around the transformation of Bella. As Eric is her maker, her physical and mental transformation is a big part of it. Also, the theme of protection is common, as Eric might feel a strong need to protect his creation, Bella. And there's always the underlying theme of how their relationship impacts the wider vampire community, whether it causes jealousy, suspicion or new alliances.
A common theme is the power of sisterhood. The stories often show how Bella and her wolf sister support each other through tough times.
One common theme is sisterly love and loyalty. These stories often show the twins supporting each other through thick and thin. For example, in many fanfictions, when one sister is in trouble, the other will go to great lengths to help.