I love Ferris -- but the sister and the principle are in no way "bad guys" for sure, despite how the film makes them out to be. But I feel it's important to look at another interpretation of the film:
I think the point of the film was also looking at how artificial society is and how we've built it up into something that's much too important and serious for its own good. You must go to school, you must learn these arbitrary subjects, and you must do well in them to move on, you must be in a formal education system till the age of 16, and you must spend the next 30 years working. The movie was about bending these hard social rules and just giving it up for a day and having fun for a day. They wanted to make that snotty waiter eat his words and Ferris wanted Cam to just lighten up because he was so stuck in social alienation (between his parents, expectations, and unwillingness to relent to Farris' shenanigans). The best point about this is made that Ferris decides to play hooky during the bust 9-5 workday when school is in, the stock market is going nuts, and everyone is working while Ferris and friends are just enjoying the day and making the most of it -- because lets face it, he could have done all that on a weekend.
The Principle was right to pursue Farris when he knew better than to think Ferris was really sick, and the sister was right for thinking that he's lofting and it wasn't fair to her who plays "the game of life" more seriously. But Farris says that it's good to just stop and take a breather every once in a while. Whether he's a sociopath is up to you, but he's apparently a nice guy to both the "jocks and the dweebs", but also, like Cam said, "if I wanted to stop you, I could have... not even the great Ferris can stop me" (or something like that).
