Well, it could change a lot. For example, Hermione might have some internal conflict about her family connection to Voldemort. Maybe she would try to hide this fact from her friends. Also, Voldemort might have different plans regarding Hermione, instead of just seeing her as an enemy like in the original story.
In this fanfiction, the relationship would likely create a whole new dynamic. Since Voldemort is Hermione's uncle, it might add a layer of mystery to Hermione's family history. It could be that Voldemort has some hidden motives related to Hermione. He might want to use her in some way, or perhaps he has some strange sense of family loyalty that the original story didn't touch on. This would also put Hermione in a very difficult position in the wizarding world, as she is related to the most feared dark wizard.
Well, in such a fanfiction, it could completely flip their relationship. Maybe Harry realizes that Voldemort has some hidden motives that are not entirely about killing him. It might be that Voldemort sees some value in Harry that he wants to use for his own gain, rather than just destroying him like in the original story.
Well, it could change Ron's initial perception of Hermione. Maybe he'd be more hesitant at first because of the cultural differences, but then grow to respect her even more as he gets to know her better. In the case of Harry, he might be more empathetic towards Hermione's experiences as someone who has also faced discrimination.
In a fanfic with Hermione as Voldemort's spy, it's a very interesting concept. First of all, Hermione is known for her intelligence and loyalty in the original series. But if she's a spy, her intelligence could be used against the good guys. She could be using her knowledge of spells and magic to deceive her friends. The plot could involve her gradually realizing the error of her ways and trying to turn against Voldemort. Or perhaps she's so deep in the spy role that she becomes a real antagonist, causing chaos and destruction among the characters we love.
Well, in such a fanfiction, Harry being Voldemort's snake might initially make their relationship more of a master - servant one. But as the story progresses, Harry could start to resist Voldemort's control, leading to a more adversarial relationship.
It makes her seem more powerful. Traditionally, Hermione is seen as the brains of the trio, but in this fanfiction, she's scarier, which implies a new level of power that goes beyond just being intelligent. It gives her an edge that we don't usually associate with her character.
Well, it changes a lot. For example, Harry would have more emotional support right from the start. Hermione, being his sister, would be more likely to be by his side constantly. So in the early years at Hogwarts, she might be able to prevent some of the bullying he faces from Snape or Draco.
Well, if Hermione is Harry's sister in fanfiction, their relationship becomes more of a family - based one. They would have a deeper, more instinctive bond. Harry might be more protective of Hermione, like an older brother would be. And Hermione could be more likely to give Harry sisterly advice, rather than just being a friend.
It changes the story by adding a new layer of heroism to Snape. In the original, his heroism was more in the background. But here, he becomes Hermione's savior from Voldemort. This could also change Hermione's view of Snape completely.
The original story has a strong theme of self - sacrifice and heroism. In the fanfiction where they are related, this could be altered. Harry might struggle more with the idea of having a connection to Voldemort. It could add a layer of internal conflict, as he wonders if he has some of Voldemort's darker tendencies within him. This could change the theme from a straightforward battle of good against evil to a more complex exploration of nature vs. nurture. Harry has to decide whether his relation to Voldemort defines him or if he can rise above it and still be the hero we know from the original story.
In the original books, Draco and Hermione were mostly enemies. But in these fanfictions, with Draco as Hermione's master, there's a power imbalance that changes things. Draco has a certain authority over Hermione which makes their interactions more tense and charged.