It's NOT What Brothers Do (HUMAN!Fontcest)
Long ago, in an alternate reality, it was monsters who were stronger, and won the war. They then went on to build civilization, and democracies above ground.
Meanwhile, our loveable characters from Undertale are now humans. After they reach the surface, they realize it's much better in the Underground, and return to their old homes, where King (FluffyBuns) Asgore passes new laws picked up from the surface world.
Sans is a 21 year old, who has a crush on his 18 year old brother, Papyrus. He was even going to admit these feelings, he decided, shortly after getting back home. But now there's a law against that kind of stuff. Will he chicken out? Or will he take the risk? And, if he does, will it be worth it, or will he and Papyrus pay the price?
((Fontcest, you no like, you no read, good day.))