This incident occurs in Chapter 26, and is actually a very important part of Huck's development into maturity and being able to take responsibility and act on his principles. It is interesting that up to this point, Huck is very much a laissez-faire observer--he does nothing to try and change what is going on around him apart from running away. His attitude is expressed perfectly when the Duke and the King come along and he does nothing, merely hoping that they will go off on their own. It is only when he stays at the Wilks and is overwhelmed by the trust and kindness of Mary Jane and her sisters that he realises that he is unable to sit back and just watch the Duke and the King rob them of their money, which is rightfully theirs:
I says to myself, this is another one that I'm letting him rob her of her money. And when she got through they all jest laid themselves out to make me feel at home and know I was amongst friends. I felt so ornery and low down and mean that I says to myself, my mind's made up; I'll hive that money for them or bust.
This is therefore a very significant part of the novel in terms of Huck's own development and growth, and what is particularly interesting is Huck's obvious development of a conscience that he is willing to act upon. This of course foreshadows his much greater decision to try and rescue Jim, even if that means he will go to hell.