Well, it would definitely change Harry's relationship with Ron and Hermione. The twin sister might become a closer friend to Hermione, sharing a different kind of bond as two female witches. And Ron might be a bit jealous at first of the new addition to their group.
In terms of the relationship with the other students, having a twin sister would make Harry seem less of an outlier. The sister could be more sociable than Harry in some ways, which would draw more students to their circle. And this could also change how the other houses perceive Harry, as his sister could act as a bridge between the houses.
If she exists, it might affect the relationship between Harry and his friends. For instance, Ron and Hermione might also become closer friends with her. And she could bring a new perspective to their group. Say she is more interested in the history of magic and spends time researching in the library, she might find clues that help them solve problems in a different way than Harry would have on his own.
His presence could speed up the journey of the Fellowship. Harry's magic could be used to quickly cross difficult terrains or overcome obstacles that the original Fellowship had to struggle with. For example, he could use a spell to create a bridge over a chasm instead of having to find a way around it.
If Harry Potter had a twin sister in fanfiction, it would likely make the story more complex. She could have been sorted into a different house, like Gryffindor or Ravenclaw. This would have given her a different set of friends and experiences. Her relationship with other characters would also be different. For instance, she might have had a closer bond with Hermione or Ron in a unique way. And in battles, she could have used her own unique skills, perhaps being more stealthy or having a special talent for defensive magic.
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.
Emotionally, it would be a huge shift. Ciel has been somewhat cold - hearted in his pursuit of revenge. His sister could thaw that ice a bit. She might encourage him to look for other ways to get justice rather than just revenge. And in relation to the other characters, they would have to adjust to this new presence. Sebastian, for instance, would now have to protect two people instead of one, and he might have to deal with the sister's different demands and personality, which could be a source of both conflict and humor in the story.
His relationship with Hermione and Ron would be strained at first. They would be confused and a bit scared. But as they see Harry resist the dark side, they would become more supportive. With Dumbledore, there might be a new level of secrecy and mistrust at the start, but Dumbledore would likely keep an eye on Harry to see how he develops. As for the other students, many would be afraid and avoid him, but some might be curious and try to befriend him to see what it's like to be around the heir of Voldemort.
It changes the family dynamic. Instead of being an only child, Harry now has a sister, which means there's more family support or potential family drama. For instance, the sister could be a target of Voldemort too, adding another layer of danger.
Well, in terms of relationships, Harry's friendship with Neville could be strengthened. Neville, who always felt a bit in the shadow of Harry, might find that Harry's new identity as the son of Bastet makes him more relatable. Because now Harry is also dealing with something very different from the normal wizarding heritage. With Snape, Snape might be even more hostile at first as he doesn't understand this new element in Harry, but later could come to respect it. And Harry's relationship with Ginny would be different too. Ginny might be in awe of Harry's new powers and their relationship could develop in a more magical and complex way.
His relationships would likely be in turmoil. His friends might notice a change in Harry and be confused. Loki could make Harry act distant or say strange things to Ron and Hermione, causing them to worry. And his relationship with Ginny might also be affected as Loki might make Harry act in ways that Ginny doesn't understand.
At Forks High School, Harry Potter would be a game - changer. His quick thinking and ability to adapt, which he developed at Hogwarts, would help him in this new environment. He might start clubs or activities that are different from the norm. For example, he could start a self - defense club inspired by his dueling experiences. His presence would also attract attention from not only students but also teachers, who would be intrigued by his different way of looking at things. And his friendship with Hermione and Ron at Hogwarts has taught him the value of loyalty and friendship, which he would bring to Forks High, making him a great friend to those in need.