This guy has been a colossal drunk and drug abuser his entire life...
I know his father, Fred Jr (Donald's brother) was. Are you saying like father like son?
To me the main motivation is that Fred Jr. was left out of the will when Fred & Donald's father died. Sister & brother of Fred Jr. are bearing false witness against their Uncle because of spite for their slight which left them not being rich, but also left them with deep-seated hatred for Uncle Donald who increased the family fortune considerably. Just another aspect of their father's shortcomings & the impacts he inflicted upon the family he had started. In other words, they turned out no better than dad. But instead of placing the blame with their father & themselves where it belongs, they prefer to blame Uncle Donald.
That was his father.