Why do the king and the duke give their inheritance to the Wilks girls in chapter twenty-five of Mark Twain's novel

This is all a part of their scam. To impress the family by their generosity and gain their unquestioning respect, they give the total amount to the dead man's daughters. As anticipated, the daughters are completely grateful, and later, when the family doctor comes and accuses the two scoundrels of being frauds, Mary Jane, the oldest, demonstrates her total faith in the King and the Duke. She returns the money to them, and instructs them, "Take this six thousand dollars, and invest for me and my sisters any way you want to, and don't give us no receipt for it"