Dumbledore would be a key figure in the reactions. If he believed in Snape's innocence, he would try to find a way to free Snape while also not alienating Harry. He might start his own investigation to uncover the truth. The other Hogwarts teachers would be in shock. Some might believe Harry is right, especially those who have always been a bit suspicious of Snape, while others would be outraged and demand an explanation from Harry.
Maybe Harry accidentally frames Weasley for something. It could be due to a misunderstanding, like a spell gone wrong that makes it seem like Weasley was involved in some illegal magic use.
In the fanfic, it could be that Snape is caught in a political struggle within the wizarding world. Harry, who is on the side of the 'good' political faction, uncovers Snape's involvement in some underhanded schemes for the opposing side. As a result, he has no choice but to get Snape arrested to protect the safety and stability of the wizarding community.
Maybe it's a corrupt Ministry of Magic official. They might use Harry's credibility to their advantage and trick him into providing false information about Weasley. They could be after the Weasley family for political reasons or because they want to cover up their own misdeeds by creating a distraction with Weasley's arrest.
One possible reason could be a misunderstanding. Maybe some dark magic was involved that made Harry believe the Weasleys were acting against him or the wizarding world in a way that seemed illegal. For example, if there was a spell that distorted his perception of their actions.
Well, it would surely strain Harry's relationship with Hermione and Ron. Ron would be extremely angry and hurt that his best friend got his family arrested. Hermione would probably be disappointed in Harry and try to figure out what went wrong. Their friendship would be in jeopardy.
Perhaps it was because of the power he amassed in the process of defeating Voldemort. In the fanfic world, power can be a very scary thing to those in authority. Harry's new - found and possibly overwhelming power could have made the existing magical power structure feel threatened. So, they arrested him as a way to control and contain that power, rather than seeing him as a hero who saved the wizarding world.
A really interesting plot twist could be that after getting pregnant, Lily loses her memory. So Snape has to deal with not only the fact that he's going to be a father but also that Lily doesn't remember their relationship. And then James steps in to help, thinking the baby is his. This would create a very strange love triangle situation. Also, what if the baby Harry has some sort of special power from the start that no one expected, which could be a major plot twist.
There may be such fanfictions in the vast world of fanfiction, but this kind of plot often goes against the original tone of the 'Harry Potter' series. It involves some rather unorthodox and out - of - character interpretations. Fanfiction can be very diverse, but this particular theme might not be widely accepted by all fans.
Hermione could be a great help. She is very intelligent and resourceful. She could help Harry find evidence against Snape. For instance, if Snape is suspected of cheating in grading, Hermione could use her knowledge of Hogwarts' grading system and records to dig up proof.