>> So if for example I know that my uncle is not paying all his taxes I'm a criminal too unless I report him? Or is it only for bigger frauds?
> Yes if the government can prove that you knew about it without telling, you can be charged with it.
That's not generally true. In the vast majority of cases, simply knowing about a crime and not reporting it is not a crime in itself. You only become liable when you aid-and-abet or join in a conspiracy to commit the crime.
However, it depends very much on the specific crime. Tax evasion may be entirely different than say selling drugs.
