Co-parenting is hard enough when the parents are the divorced biological mother and father of the children. It is almost a given that parenting styles will be starkly different, likely one of many factors leading to the divorce in the first place. Now, let's mix in one or two more personalities ... the new spouses of each bio-parent, likely with parenting styles of their own, and you've created a recipe with all the necessary explosives for potential parental disaster. If at all possible, stepparents should heed the following rules:
While you may treat your stepchildren as your own, loving them unconditionally as you would your own biological children, the fact is that you have a special and distinct role in the family, which is separate from mom and dad. Recognizing and embracing this different role will help define you as a parental figure, while avoiding blurring the lines and confusing the children.
For more ideas on parenting consider Bil Lepp's Muddling Through: Perspectives on Parenting or Tim J. Myers Glad to Be Dad: A Call to Fatherhood.