Complex sentence: Most of the novel focuses upon his relationship with Fudge, the fact that he is the perceived source of his troubles and that his parents seem to let Fudge get away with anything and everything... something that Peter would later realize was not always true.
Split into simpler sentences: Most of the novel focuses upon his relationship with Fudge and his family, and the fact that Fudge is the perceived source of all of his troubles. His parents usually seem to let Fudge get away with anything and everything, something that he would later realize was not always true.