It takes time to build a positive step parenting relationship. Hopefully, good preparation went on before the actual introduction and expectations are defined and understood.
1.) Discuss with your spouse what his/her expectations are of you as a step parent.
2.) Be clear about the role and how it looks to you, your spouse and children.
3.) Have clear bounaries with the children and their parent.
4.) When you feel overwhelmed and frustrated ask yourself "What is best for the children" don't just react to what is being said and done, take time to think about your response.
Remember your family is evolving and needs of the children and parents shift and change so flexibility is important. Making changes from time to time doesn't mean mistakes were made, it means you are making room for the growth of the family.