It's completely inappropriate. Undertale is a game that is enjoyed by a wide range of audiences, including younger players. Writing such fanfiction not only distorts the relationship between the characters but also can lead to a bad reputation for the fan community. We should encourage creative and positive fan works that uphold the values of the original game.
One common theme is the exploration of their relationship in the Underlust AU context. Since it's an AU with a focus on different interpretations of the characters' personalities and relationships, fanfictions often delve into how Papyrus and Sans interact with each other romantically or platonically in this altered world.
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.
You can try looking on Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Undertale and its AUs like Underlust. Just use the search function and type in 'Underlust Papyrus x Sans' to see what's available.
Another option is to join Undertale - related forums or communities. People there often share and recommend good fanfictions. For example, on the Undertale subreddit, you can post a request for undertale sans x underswap papyrus fanfictions and I'm sure the community will be happy to help.
A great idea could be a story about their daily adventures in the Underground. For example, they could be on a quest to find a rare ingredient for a new recipe Papyrus wants to try. This would focus on their brotherly bond and Papyrus's enthusiasm for cooking, while Sans tags along with his laid - back attitude.
Well, it could be about Gaster suddenly reappearing in the lives of Sans and Papyrus. Maybe he has some important knowledge or a mission for them. For example, he might need their help to fix something in the Underground that's going wrong.
Obviously, Sans and Papyrus are the two main characters who we already know well from the original Undertale. They have distinct personalities that will play a big role in how they interact with Gaster. Gaster, on the other hand, is the enigma. His presence will likely drive the story as Sans and Papyrus try to figure out who he is now and what he wants, so he's also a main character in this fanfiction.