Things can definitely get confusing in "I, Claudius" because there are just so many characters! For a good "I, Claudius" family tree, check out this website: http://www.geocities.com/yvain.geo/claudius.html Although some of the minor family figures are missing, the tree includes all of the most important characters and is useful for figuring out all of the familial relationships.
As far as the vocabulary goes, the current Classic Note for "I, Claudius" does not divide the vocabulary by chapter. However, if you skim through chapters 21-34 and highlight some of the more difficult words, chances are that I provided a definition!
Good luck and happy reading!