In fanfiction, often the threats come from the remnants of the Death Eaters. They would see Harry's sister as a way to get revenge on Harry. They might try to curse her or use dark magic on her. Then there are the dark forces that are always lurking in the wizarding world. These could be ancient evils that are awakened and target Harry's family. Additionally, the wizarding world can be a dangerous place for those who are not well - versed in magic. So, Harry has to protect his sister from making mistakes that could endanger her, like using magic in the wrong way or getting involved with the wrong people.
The most common threats are related to the power struggle in the wizarding world. Since Harry is a key figure, his sister becomes a target. Fanfictions often depict threats from political opponents who want to use his sister to gain an edge. There could also be threats from the Muggle world if the secret of magic is in danger of being exposed because of her. Moreover, there are magical diseases or curses that could befall her. Harry has to be on guard against all these threats, using his knowledge of magic, his friends, and his own courage to keep his sister safe.
Dark wizards are a common threat. Voldemort's followers might target his sister to get to Harry. They could try to use her as a hostage or a way to lure Harry into a trap. Another threat could be magical creatures that are out of control. In some fanfictions, there might be a basilisk or a dementor loose near where his sister is. Also, there could be jealous classmates or rivals who might try to harm her out of envy for Harry's status.
One way Harry Potter could protect his sister in fanfiction is through his fame and influence. People in the wizarding world respect and fear Harry. He could use this to his advantage. If anyone tried to harm his sister, he could make it clear that there would be serious consequences. Also, he might have some special magical artifacts that he could use to protect her. For instance, the invisibility cloak could be used to hide her when needed. Harry's connection with powerful wizards like Dumbledore could also play a role. He could ask for help from them if the situation demanded it.
One of the typical situations is when Harry is on a quest or mission that is fraught with danger. Bill may accompany Harry, using his expertise in dealing with various magical entities. For example, if they are exploring an ancient tomb filled with traps and dark magic, Bill can use his knowledge of curse - breaking to disable the traps and protect Harry from any lurking dark forces. He also might use his ability to communicate with magical creatures to enlist their help in safeguarding Harry during their journey.
One common theme is redemption. Slytherins are often seen as the 'bad guys' in the original series, so protecting Harry gives them a chance at redemption. For example, they might be trying to make up for past wrongs or the actions of other Slytherins.
One common theme might be the struggle against societal norms. Since this kind of relationship is not acceptable in the normal world, the fanfiction could explore how the characters deal with the outside world's judgment. But this is still a very controversial theme.
Obviously, Harry Potter himself is a central character. His sister would be the new addition. Hermione and Ron are likely to be there as well. They are Harry's best friends in the original series and would probably play important roles in the fanfiction too. Dumbledore might also appear, as he is a very important figure in the wizarding world and could guide the sister in her magical journey.
One common plotline is that Charlie gets wind of a plot against Harry before it fully unfolds. He then races back to Hogwarts or wherever Harry is and sets up a protective perimeter around him. This often involves a combination of magical spells and the use of his dragon - taming abilities in a creative way to keep the bad guys at bay.
Often, there is a part where Naruto has to prove himself to his sister or gain her trust. Since she may not know him well or may be scared at first. Also, Naruto usually has to overcome his own limitations in power to be able to protect her effectively. For example, he might have to learn a new jutsu or master an existing one in a short time. And there is always the emotional aspect of the bond between them growing stronger as he protects her from different perils.
Well, often there's the theme of adaptation. Given that Lemon is not part of the original story, the fanfiction might focus on how she adapts to the wizarding world. Family secrets could also be a theme. Maybe there are secrets about their family that they uncover together. And of course, there's the theme of growth. Both Harry and Lemon could grow as characters through their experiences, whether it's dealing with the challenges of Hogwarts or facing the dark forces in the wizarding world. There may also be themes related to love and acceptance within the family unit.
Well, family loyalty is a huge theme. Since they are twins, they have this innate bond that is explored in many fanfictions. Also, the theme of shared destiny. They are both part of the wizarding world and might have a shared role in fighting against evil. Additionally, there's the theme of growing up together and how their relationship changes as they age, from simple sibling love to a more complex and deep connection.
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.