Often, in such fanfictions, the theme of identity can be prominent. The Salvatore sister might be redefining herself through this marriage in an original way. There could also be a theme of power dynamics. If the 'original' she is marrying has some sort of power, how does that affect their relationship and her within the family? Also, the theme of acceptance, both from society within the fictional world and from her own family, is likely to be explored.
In Salvatore sister married to original fanfiction, a recurring theme is probably the struggle between the old and the new. The sister's marriage to an 'original' might bring new elements into the Salvatore family's world that clash with their traditional ways. Another common theme could be loyalty. Is the sister's loyalty to her new partner conflicting with her loyalty to her family? And of course, there's the theme of destiny. Is this marriage something that was fated, or is it a choice that goes against what was expected in the story's universe?
One common theme could be love and sacrifice. Since it's about a marriage, there might be a story of the Salvatore sister sacrificing something for the person she marries in the original context. Another theme could be family reactions. The Salvatore family's response to the sister's marriage would likely play a significant role in the story.
I'm not sure of specific 'Salvatore sister married to original' fanfiction off the top of my head. But you could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. They have a wide range of fanfictions and you might be able to find what you're looking for by using relevant tags and keywords.
One key element is the cultural clash. The Salvatore sister comes from a certain vampire - hunter background, while Finn has his own vampire - witch heritage. Their different upbringings can lead to interesting conflicts and resolutions. Another element is the family reactions. The Salvatore brothers' reactions to their sister marrying Finn would be a big part of the story, as well as Finn's family's take on it.
A common theme is love triangles. Since Katherine has a history with the Salvatores, an OC as their sister can easily get caught up in the complex web of emotions. For example, she might be in love with someone one of the Salvatores also has feelings for.
A fanfiction might revolve around the Salvatore sister and Finn's journey together. After getting married, they decide to travel to different places to find a way to combine their different magical heritages. Along the way, they meet various characters who either help or hinder them. The Salvatore sister uses her wits and skills learned from her brothers, while Finn uses his knowledge of his own kind. They face many trials but their love for each other keeps them going.
Love and relationships are important themes too. She may fall in love with a human, a vampire, or another supernatural being. This creates a lot of drama as she has to balance her feelings with the secrets of her true nature. In some fanfictions, her relationship with her father also evolves as they face various challenges together, strengthening or straining their bond depending on the plot.
One common theme is betrayal. Damon often gets hurt when someone he trusts, like a friend or a lover, turns against him. Another theme is loss. Whether it's the loss of a loved one, his powers, or his status, it causes him a great deal of pain. And of course, the theme of self - loathing is also there. When things go wrong, Damon tends to blame himself and that hurts him even more.
The theme of isolation is also common. The victims of Damon's abuse may isolate themselves from others due to fear or shame. And for Damon himself, his abusive actions can lead to his own isolation from the vampire community or his friends. Fanfictions may also explore how he copes with this isolation, whether he tries to break free from it or sinks deeper into his dark ways.
One common theme is friendship. The OC often forms deep friendships with both the Salvatore brothers and Hope Mikaelson. Another theme is self - discovery. The OC usually has to figure out their place in this world filled with vampires and other supernatural beings.
One common theme is the struggle for power. With the powerful Mikaelson family and the complex Salvatore brothers involved, the OC often gets caught in the middle of power plays.
One common theme is forbidden love. Since Stefan is a vampire, his relationships are often frowned upon in the world of the story. Another theme is sacrifice. He might have to sacrifice his own desires or safety for the one he loves. Also, redemption can be a theme, where his love for someone makes him want to be a better vampire.