As I’ve written many times before, all languages change over time because of common usage. I’m sure you often hear people use the word hysterical to refer to something funny. That is common usage and will, in time, become a standard definition. For now, though, hysterical refers to uncontrolled and extreme emotion. Picture an audience of teenage girls in the 1960s seeing the Beatles: they were hysterical with excitement.
Hilarious simply means something extremely funny: I find most Mel Brooks movies hilarious. However, I manage not to become hysterical.