To what extent does fate determine the events in the book?
The interesting thing about the novel Holes is that one could make an argument that the plot is driven by fate OR free will. Certainly the idea of Stanley's family curse does seem to tie everything up nicely, with Stanley carrying Zero up the mountain preluding many things going right for the Yelnats family, after a long period of things going wrong. Additionally, even though Stanley and his family claim to not really believe in the curse, on some level it could be said to influence the choices they make. After all, as the saying goes, "whether you think you can or think you can't, you're right." So there is definitely a case that all the events in the novel were fated to happen the way they did.
However, there is an equally compelling case that the curse and the family's fate was just an illusion. Consider, for example, how many random coincidences occur that don't have anything to do with the curse: Clyde Livingston and Trout Walker have the same rare foot disease, Zero and Stanley find the Mary Lou and peaches and onions to survive, etc. Perhaps these are all in the novel to show how full of coincidences and unexplainable life is, rather than to indicate a fateful event.
In the end, Sacher leaves it to the reader to interpret the story and the role fate plays.