There are some. You might want to check fanfiction.net as well. However, the quality can vary a lot. Some fanfictions focus more on the relationship development between Underswap Papyrus and Underfell Sans, while others might include more action or adventure elements related to their world.
The clash of personalities. Underswap Papyrus is usually quite positive and energetic, while Underfell Sans has a more edgy and guarded nature. This contrast can lead to really engaging interactions in the fanfictions.
The fan base loves these characters so much that seeing them interact in new and unexpected ways is always a treat. The relationship between them can be explored in so many different ways, from friendship to rivalry. And the unique art styles and interpretations that often come with these fanfictions add to the overall appeal. There are also some really emotional stories that can be told about them finding common ground or learning from each other, which really tugs at the heartstrings of the readers.
There are many alternatives. You could look for fanfictions that focus on their platonic relationship, like them going on a quest together in the Underground. Another option is fanfictions that explore their individual character growth outside of a romantic or inappropriate context.
I don't recommend seeking out 'lemon' fanfiction, especially for undertale sans x papyrus. Their relationship in the original game is more of a brotherly one. If you want to explore fanfiction about them, there are plenty of non - 'lemon' ones that focus on their adventures, character development, and the unique bond they share in the Undertale universe.
There's also 'Sibling Rivalry Reimagined' which delves deep into how their personalities from their respective universes interact. It has some really interesting character development for both Sanses. It shows how their differences can lead to both conflict and unexpected understanding. The author does a great job of portraying their emotions and the way they communicate, which is quite different from the original game's interactions.
Frisk would be a likely candidate. Since Frisk is the main character in Undertale and has a very adaptable personality. They could approach Underfell Sans with an open mind and try to understand him.
One popular one might be 'A Tale of Two Skeletons'. It delves deep into the relationship between undertale sans and underswap papyrus, exploring their different personalities and how they interact in a unique AU setting.
Writing fanfiction about incestuous or inappropriate relationships like that goes against the positive and family - friendly nature of the Undertale game. Such content is not acceptable in the general Undertale community, so I don't think there should be any available or promoted fanfiction of this sort.
Well, first of all, they are brothers. Incestuous relationships are considered unethical in most cultures and moral standards. In the context of Undertale, their relationship is one of brotherhood and friendship, not a sexual one. Fanfiction should enhance the positive aspects of the original work, not distort it with such inappropriate content.