Hi zzxxee,
No-one has answered yet, so I'll try! I don't have step-children, but have lived with my partner for 4 years, so he is step-dad to my kids. I introduced them very gradually, but my youngest did test him out a bit. My son was 3 at the time and finds changes very difficult to adapt to. My partner has always been very patient and kind to him and they get on very well now.
I also had a step-dad from the age of 7, but don't remember being jealous. He is my dad to all intents and purposes.
Are these your step-children? Just wondering what ages they are, as that will make a difference?