Well, without having read it, it could potentially be about a twin that has been hidden or forgotten in the Twilight saga. It could explore the concept of how this twin's existence might change the dynamics of the story. For example, if it's a vampire twin, it could introduce new power struggles or love triangles within the already complex world of Twilight. It might also deal with themes of identity, as the twin tries to find their place among the known characters.
The main character is surely the 'other twin' as the title suggests. In addition, we can expect to see the presence of Twilight's main protagonists such as Bella Swan. Her relationship with the vampires, especially Edward Cullen, was a central part of the original story. So, it's likely that in this fanfiction, their presence will be felt as the 'other twin' might be intertwined with their story. There could also be the werewolves, led by Jacob Black, if the story involves the conflict between vampires and werewolves which was a significant aspect of the Twilight saga. So, in summary, the 'other twin' along with Bella, Edward, and potentially Jacob and other related characters are likely to be the main characters in this fanfiction.
First, you can start with his appearance. He could look similar to Twilight but have some distinct features, like a different color in his mane or tail. In terms of personality, he might be more outgoing than Twilight. For example, he could be the type to easily make friends with everypony he meets. This would contrast with Twilight who can be a bit more reserved in new social situations.
The relationship between Renesmee and Jacob could be affected. Jacob might feel a different kind of bond with the twin. He would have to balance his loyalty to Renesmee and his new connection with her sister. This could create some jealousy or confusion among them, adding a new layer of drama to the story.
You can start by giving her a distinct personality. If Twilight is studious and a bit reserved, make the sister more carefree and impulsive. Maybe she has a passion for art or music that sets her apart. Her appearance could also be different in a subtle way, like a different shade of mane or a unique cutie mark.
One possibility could be that the twin has different powers than Renesmee. For example, while Renesmee has her unique gift of showing people her thoughts through touch, the twin could have the power to control the elements. This would create a lot of interesting situations within the Twilight universe, like how the Cullens would deal with this new power in their family and how it would affect their relationships with other vampire clans.
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.
A common trope is the twin having a different supernatural ability. For example, she might be able to control the elements while Bella has her mind - shield. Another is the love triangle, but instead of Edward - Jacob - Bella, it's between different characters involving the twin. Often, the twin also has a secret she's keeping from Bella which drives the plot.
Twilight Sparkle in other world fanfiction often explores the concept of Twilight being transported to a different realm. This can lead to all kinds of interesting scenarios. For example, she might have to adapt to new magic systems, meet strange creatures, and face challenges that are unique to that world. It's a great way for fans to expand on the My Little Pony universe and see their favorite character in new and exciting situations.
One of the most interesting plot twists could be if Rosalie's twin had a hidden power that no one expected. For example, perhaps the twin could communicate with animals in a way that even the werewolves in the Twilight universe couldn't understand. This would add a whole new dynamic to the story as it could potentially change the balance of power between the vampires and other supernatural beings.