First, you have to define what you are not – you are not their mother, father or friend (because they certainly have enough friends). You are a moral compass. You are a person that you want them to look up to and model after. You can certainly be someone they can confide in, but you have to make it clear that if they tell you anything that is considered life-altering, life-threatening or against the law (even family rules), that you must share the information with their parent.
As with any new situation, the key is to always be yourself. In time, if you parent your step children appropriately, you will find your relationship will grow and improve until the time the children reach a place where the distinction of step is not even necessary – for you or for them.