What qualities of the Wilks girls allow them to be duped so easily in The Adventures of Huckleberry Finn?

Pretty much everything about the Wilks sisters make them easy prey for those who might want to take advantage of them.  They are completely trusting, accepting the duplicitous duke and king as their long-lost relatives without question.  They are also loyal, standing by the duke and king even when "a big iron-jawed man" named Robinson, the family doctor, warns them "as a friend, and an honest one, that wants to protect (them) and keep (them) out of harm and trouble, to turn (their) backs on (the) scoundrel(s), and have nothing to do with (them)".  Mary Jane in particular is appalled that someone should question the integrity of the duke and the king, her departed father's purported brothers, and defiantly demonstrates her loyalty by giving them a bag of money, telling them to "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" (Chapter XXV).

Of all the sisters, Joanna, the "hare-lip", is the least naive and the most discerning.  She catches Huckleberry weaving a web of lies, and although she is not exactly sure what is true and what is not, she is aware that he is not telling her the whole truth.  Her sisters, however, especially Mary Jane, berate her for expressing her doubts to Huckleberry, and in doing so reveal the complete spirit of selfless generosity that underlies their approach to life and in effect makes them so susceptible to the wiles of malicious others.  When Joanna points out that Huckleberry is, in fact, telling lies, Mary Jane responds,

"I don't care whether 'twas little or whether 'twas big, he's here in our house and a stranger, and it wasn't good of you to say it.  If you was in his place, it would make you feel ashamed; and so you oughtn't to say a thing to another person that will make them feel ashamed...It don't make no difference what he said - that ain't the thing.  The thing is for you to treat him kind, and not be saying things to make him remember he ain't in his own country and amongst his own folks" (Chapter XXVI).