Love and friendship are also common themes. The baby sister may find love among the supernatural beings or form strong friendships. For example, she could become friends with some of the werewolves while also having a connection with the Cullens. It adds a new layer to the existing relationships in the Twilight world.
The baby sister might also attract the attention of other supernatural beings. If she has an alluring scent like Bella, it could draw vampires or werewolves towards her. This would force Bella to take on a protective role and might lead to new alliances or battles in the Twilight universe. For example, the Cullens and the werewolves might have to work together more closely to keep the baby sister safe.
There is a fanfiction called 'Bella's Little Sister's Adventure'. In this story, the baby sister embarks on her own journey in the Twilight world, facing challenges and making unique friendships. It's a great read if you like seeing a different perspective in the Twilight universe. The author does a good job of integrating the new character into the existing world and storylines.
The baby sister in fanfiction could also have special abilities. Maybe she has a latent psychic ability that she discovers as she grows up. Bella, with her knowledge of the supernatural, could help her sister understand and control these powers. This would create a strong bond between them, different from a typical sister relationship, as they navigate the challenges of these unique abilities together.
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.
Love is another prevalent theme. Bella and Edward's love extends to their love for their child. And there's also the love of the Cullen family for the baby. Additionally, the theme of the baby's special abilities or powers is common. Because in the Twilight world, the baby is a unique hybrid, fanfictions often explore what these powers mean and how they affect the baby's life and the people around it.
Family also often plays a big role. Some 'baby bella twilight fanfiction' might focus on Bella's family relationships when she was younger. It could explore how her relationship with her parents differed before the events of the main Twilight story, and how those early family bonds affected her development.
The sister could have a big impact. Suppose she has a gift of reading minds like Edward. She might be able to tell if Edward has any hidden motives towards Bella. If she senses something wrong, she'll surely try to break them up. But if she approves of Edward, she could actually be a great confidante for both of them, and help them communicate better when they have misunderstandings.
There could be other minor characters from Forks that get more spotlight in these fanfictions. For instance, the classmates of Bella's little sister who start to notice that there's something strange about her. They might become curious and start to dig deeper into the secrets of the town, which then draws in the vampires and werewolves.
I really liked a fanfic where Bella's twin sister was a werewolf. It added a whole new layer to the story. Instead of being drawn to the Cullens like Bella, she was more involved with the werewolf pack. It explored how she tried to keep her identity a secret from Bella at first, which led to some really tense and exciting moments.