I have been a step-parent. I have found only one successful method of discipline that works with a step-child (yes, it really works if adhered to strictly!) Here it is:

The birth parent needs to set boundaries, limits, and rules. The step-parent's role should be that of 'the enforcer' when the birth parent isn't available, prefacing each statement with "Your father/mother says _________ and I am here to enforce that."
It's the ultimate peace-keeper and it works.