How can I report someone living in this country illegally for over 20years?

Sure you can..

How can I report someone living in this country illegally for over 20years?

The immigrations and Naturalization offce would be a good start.

Here is a link with the numbers to call depending on where you live.
INS Contact Info For Reporting Illegal Aliens (http://www.criminalgovernment.com/docs/rel/aliens.html)

Unless they are breaking some law, don't expect anyone to get excited, There is not immigration officers to hardly investigate the illegals that a danger.

Why exactly do you want to report them, what did they do ?

If I had a nickel for every one of these calls I'd have a lot of nickels.

And what do you gain by getting this person deported? And the next question will be why it took you 20 years to file the report.

And that's only if someone cares enough to talk to you.

This is under "Other Family and People" and NOT immigration so I will guess that it's somehow tied in to your other thread about your parent's estate and your problems with your brother's handling of the estate.