They are likely to be horrified. James, for example, would probably be extremely angry that his friend had to endure such a thing. He might immediately vow to protect Sirius from any further harm. Remus, being more empathetic, would be there to comfort Sirius and offer his support in a more quiet and understanding way. Peter, though not as strong - willed as the others, would still be shocked and follow the lead of James and Remus in trying to make Sirius feel better.
The marauders' reaction would be quite complex. They might feel a sense of nostalgia as they read about the world they once knew, but through a different lens in the fanfiction. They could also be a bit critical. If the fanfiction didn't seem to respect the essence of the original story, they might be quick to point it out. However, if it was well - written and added new depth to the story, they could end up really enjoying it and even discussing it at length among themselves.
Remus might be a bit more reserved. He'd approach the fanfics with caution, but he'd also be touched by the stories that explored his struggle with being a werewolf in more detail. He might find comfort in knowing that fans understood his inner turmoil to some extent.
They might be very amused. The marauders are known for their sense of humor and the 'A Very Potter Musical' fanfiction is often full of comical and over - the - top elements. So they'd probably have a good laugh at the silly situations and the creative liberties the fanfic takes.
They would probably be very curious at first. But as they read more, they might get annoyed if the fanfiction doesn't stay true to their essence. For instance, if a fanfiction portrays Sirius as a coward, which is completely against his real character, they would be offended. However, if they find a well - written fanfiction that respects their characters, they might enjoy it.
He is often very suspicious. He might immediately think it's some kind of trick by the Marauders or their descendants. He'll likely try to figure out how to use it to his advantage against those he dislikes.
They would be shocked and angry. James might immediately want to confront Sirius' family. Remus would be empathetic and try to comfort Sirius. Peter, though a bit cowardly, would also show concern.
They might be shocked at first. I mean, seeing their own stories being written in a different way by fans would be quite unexpected. Sirius might find it really cool and be eager to read more, while James could be a bit more critical, thinking about how accurate the portrayal is. Remus would probably be more laid - back and just enjoy the different interpretations.