This episode that occurs in Chapters 24-30 in the book is another indication of how Huck Finn develops as a character as he works against the dastardly scheme of the Duke and the King to fraudulently disinherit the money of the heirs of Peter Wilks.
It is when Huck experiences the kindness and generosity of Mary Jane and Susan, who stops Joanna (correctly) interrogating Huck on the suspicion that he is lying that he begins to think again about letting the girls becoming the latest victims of the Duke and the King. Note what he says:
I says to myself, this is a girl that I'm letting that old reptle rob her of her money!... And when she got through they all jest laid theirselves 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.
The girls' defence of a total stranger, as well as their appeal to their sister fro an apology, impresses Huck enough to realise that he cannot allow the Duke and the King to take advantage of these kind and gentle young girls. He decides to help the sisters. Huck's disgust at the Duke and the King demonstrates his continuing maturity. Early in the novel, Huck's morals were none too firm, but as he rafts down the river, he gets to know Jim, and has to deal with the con men, Huck is not only able to distinguish right from wrong, but he is beginning to stand up for what he believes in.