I believe that you are allowed to be included on your mother's PR if you were on her 457 application. However, this is a recent development & maybe someone else will come & help with a link to the new regulations for you. Meanwhile,if you're only talking about a few months between the date your course ends & the new application, maybe you could enrol in a short course to ensure you are still seen as dependent? Just to be fail-safe. You could still work part-time.