Remember that if you apply for PR for your step-son he will most likely have to do national service, but I suspect that it is unlikely that he'll be granted PR under family ties once he is older than 18 so I guess you'll have to consider the various options.
For your wife there is also this new visa that allows her to work. We haven't heard much about it on the board though so if you decide to apply for it it would be useful to hear your experiences.