Okay. So what other characteristics of insanity are in evidence with you?
If the answer is "none," then the only reasonable conclusion is that you are NOT insane, but said lib-in-law lacks the intellectual horsepower, or personal integrity, to admit you might have more on the ball than they do, and so resort to insults to assuage their feelings of inferiority.
Fear - “what if he’s right?”
That’s what drives the anger in most cases.