The appeal lies in the potential for new storylines. In the original Harry Potter world, we saw a lot of their relationship but not as twins. Fanfics can create a whole new set of stories. For example, they could have a special twin - like magic that they discover. Or they could be on a mission to find out more about their family heritage together. It also allows fans to delve deeper into their emotional connection, like how they would comfort each other during tough times, which is really fascinating for readers.
Well, it's the combination of their personalities. James was brave and a bit of a prankster, and Sirius was rebellious. When they are like twins in fanfics, their adventures together become more exciting. We get to see how they handle different situations as a unit, whether it's dealing with Slytherins or facing the dark arts. Also, it's about exploring their backstories more deeply. Maybe there are secrets about their families that are revealed when they are thought of as twins.