The sister could start to question her relationship with Bella. If Bella is being so forceful, the sister may wonder if Bella really respects her as an individual. She may also try to find ways to avoid the vampire fanfiction stuff, like making excuses or hiding when Bella tries to involve her.
Maybe Bella has a warped sense of what's good for her sister. She could think that the vampire fanfiction world offers a form of escapism or a different perspective on life that she thinks her sister needs. It could also be related to a relationship she has with a character in the fanfiction. If there's a vampire character she admires or is in love with, she might want her sister to be part of that world so that she can better understand Bella's feelings towards that character.
One of the main characters could be a villainous vampire. This vampire could be after the sisters for some reason, like their unique powers or because they pose a threat to his own plans in the vampire world. Also, Bella's father Charlie could be a significant character. He might start to suspect that something is different about Bella and her sister, and his reactions and attempts to figure out the truth could be an important part of the story.
It could be because of a misunderstanding. Perhaps there was a miscommunication or an incident where the sister was wrongly blamed for something, and Bella held onto that grudge. Say the sister accidentally broke Bella's favorite item but Bella thought it was done on purpose, which then festered into hatred in the fanfiction world.
Her relationship with Bella would likely become more complex. As a vampire, she would have different needs and instincts. She might be more protective of Bella in a way that Bella doesn't understand at first. For example, she could sense danger towards Bella that a human wouldn't notice. But at the same time, Bella might be a bit afraid of her sister's new vampire nature, which could create some tension between them.
Bella might initially be terrified. She is a human being kidnapped by a vampire, after all. She could struggle and try to escape, using her wits and any available resources. However, since she has feelings for Edward in the original story, part of her might also be conflicted, wondering if there is a good reason behind his actions.
She could be very sad. After all, Naruto is likely an important person in her life. She might feel abandoned and start to question why Naruto left her.
At first, Bella might be shocked and resistant. She could be filled with a sense of disbelief that such a thing is being forced upon her. After all, in the Twilight universe, Bella has her own feelings and desires, especially when it comes to relationships. Forced marriage goes against her free - willed nature. So, she would probably try to find a way to oppose it, perhaps by seeking help from her friends or looking for a loophole in whatever is forcing this marriage upon her.
She might feel guilty. Since they are siblings, she could be aware of Naruto's neglect and try to make it up to him in small ways, like sharing her food or secretly leaving him little gifts.
Tommy might be excited at first. He could be looking forward to having a new playmate. But then when he realizes she can't really play like he does right away, he might get a bit frustrated.
Percy might be initially very confused and hurt. He may try to find his sister and understand why she has left him. He could also become very angry and feel betrayed, which might lead him to take some rash actions.