I really like a fanfic called 'Sibling Secrets'. It shows how Hermione and Sirius interact when they find out they're siblings. Hermione uses her intelligence to dig deeper into their family history, while Sirius, with his experience, protects her from those who might want to use this new - found relation against them.
Well, Hermione being James' adopted sister would likely make her closer to Sirius. Sirius might see her as a little sister figure too. He could be protective of her, much like he was with James. They might share a special bond over their love for James and their shared experiences within the wizarding world.
She could have a very different relationship with Harry. Instead of being just friends, she might be more like an older sister figure, using her family knowledge to guide him. With Ron, their relationship could be more complex. Maybe there would be some jealousy from Ron as Hermione would have a more prestigious family background. And with the other members of the Black family, she would have a family bond. Some of the Black family members who were against Voldemort could be her allies in a more personal way.
Well, Hermione being James' twin sister would make her and Sirius have a tight - knit relationship. Sirius would be part of her family in a way. He'd be there for her during her Hogwarts years, and they could bond over their shared love for James. Sirius might also teach Hermione some of the more secretive and dangerous spells that the Marauders knew, and Hermione would keep Sirius in line with her logical thinking and rules - following nature, creating a balanced and loving relationship.
One popular story might be 'Forbidden Bonds' which explores a unique relationship between them. It delves deep into their characters and how they might interact in a different way than in the original series. The author creates a lot of tension and drama, making it an engaging read.
Often, it begins with Hermione trying to help Sirius deal with his post - Azkaban trauma. She uses her knowledge and compassion to reach out to him. As he heals, he starts to see her in a new light. Their relationship then grows as they start to understand each other better. The lemon parts usually are used to explore their more passionate and private moments, which are often written in a very tender and intense way, adding a new dimension to their relationship.
If Hermione was Sirius' sister, it would add a whole new layer to the story. Hermione would have access to Sirius' knowledge and resources much earlier. Their combined intelligence and power could have changed the course of the battle against the Death Eaters. Also, it might have affected Hermione's relationships with the other characters. She could be more involved in the Order of the Phoenix activities right from the start.
In a Hermione and Sirius family fanfiction, Hermione might end up marrying into the Black family. Since Sirius was an important part of the Black family before his estrangement, Hermione could find herself involved in the family's complex politics and traditions. She could use her knowledge of magic and her Muggle - born perspective to challenge some of the old - fashioned ideas within the family. Sirius, on the other hand, could be like a mentor to Hermione, teaching her about the more secretive and dangerous aspects of the wizarding world that she might not have known otherwise.
One possibility could be that Sirius sees Hermione's intelligence and determination and decides to take her under his wing. Maybe he realizes her potential and wants to protect her from the dangers in the wizarding world that he has faced himself.
There could be a fascination with power dynamics. Sirius is an older and more experienced wizard in the Harry Potter world compared to Hermione. Fanfic writers may be drawn to the idea of how his knowledge and power could interact with Hermione's intelligence and determination, thus leading to the concept of him dominating in the fanfiction. Also, it might add an element of unexpectedness or novelty to the relationship between two such different characters.