How can one deal with a father who openly commits sins and is suspected of dealing with usury, while fearing that the money earned might be unlawful?

A Muslim must think well of other Muslims and interpret their actions in the best possible light, especially a child with their parent, for Allah Almighty says: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." Doubting a parent's commission of grave sins or involvement in usury are serious matters, but declaring them without verification is even more dangerous because it constitutes slander, and slander is one of the seven destructive sins.