Sounds like sour grapes to me � that his ex-wife & child want (expected) more from your Godfather�s will. I would agree with Androcles, that they would need very reliable evidence that he was not of sound mind for it to have a chance of success.
Saying that you agree to give someone everything in your will, and you don�t � does not alter the validity of the content of the will (even if the conversation were recorded and/or witnessed by others), or whether the verbal agreement was made before or after the will was made.
If his ex-wife does indeed take the matter before a court, I would expect her claim to be rejected fairly quickly once no reliable evidence of your Godfather being of unsound mind, could be produced.