No. Writing 'smut' (pornographic or sexually explicit) fanfic involving characters from the Harry Potter series is inappropriate. The Harry Potter franchise is aimed at a wide audience including children and young adults, and such content goes against the family - friendly and magical essence of the original work.
One of the main themes is protection. Harry might feel the need to protect his sister from the dangers that lurk in the magical realm, like the dark wizards. Friendship can also be a theme as the sister might make new friends at Hogwarts, and Harry has to deal with his feelings about that. Also, self - discovery is common. The sister may be finding out who she really is and Harry is there to support her in that journey.
One common theme is family loyalty. Since they are siblings, the stories often focus on how they support and protect each other. Another theme is the discovery of self. The sister, who may have been hidden or in the background, often has to figure out who she is in relation to Harry and the wizarding world. Also, the theme of facing challenges together is prevalent. They join forces to take on the threats in the wizarding world, just like Harry did with his friends in the original series.
Ron and Hermione are likely to interact with them. Ron might be a bit more protective of the sister, while Hermione would probably study and share knowledge with her. They could all be in the Gryffindor common room, discussing spells and strategies.
One common theme could be family. Since Lily is related to Harry in the original story, adding a sister character might explore different family dynamics. Another theme could be protection. Harry might be in a position to protect Lily and her sister. And there could also be themes of loyalty among the characters.
Yes, there might be some fanfics out there that involve Sigyn and Harry Potter. However, it's not a very common crossover. You can try searching on major fanfic platforms like Archive of Our Own or Fanfiction.net. Just keep in mind that the quality and content can vary greatly among different fanfics.
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.
One common theme is the struggle against the Slytherin stereotype. The sister character often has to deal with the perception that all Slytherins are evil or dark - wizards - in - training. For example, in many fanfics, she might be really kind - hearted but is constantly judged by others because of her house.
The twins themselves are often great characters in these fanfics. Their mischievous nature and loyalty to their sister add a lot of fun and heart to the story. The sister, of course, is a central character with her own unique personality and abilities.
One popular 'Harry Potter sister fanfic' is 'The Lost Potter Sister'. It often features a long - lost sister of Harry Potter who discovers her true identity and her connection to the wizarding world. Another one could be 'Potter's Hidden Sister', where the sister has unique magical abilities that set her apart from the other characters in the Harry Potter universe.