As a child whose Father had to be forced to see me after my parents separated when I was 2, I would say it very very much depends on what kind of person and Dad he is/was.
If he is, as PP says, embarrassed about the lack of contact and doesn't know how to start putting it right but is essentially a good, kind person and capable of, over time, becoming a good role model that you would want in your DD's life then I would say maybe try to see if you can help facilitate the start of a relationship.
If, however, you split up because he wasn't a nice person, if he is incapable of being the Dad your DD deserves, then I would say leave him to it. I speak from experience, my F did end up keeping in touch, I saw him every 3rd weekend until I was 14. He never showed any interest in me, was emotionally abusive (I realise now) and when I was 14 became physically abusive. My DM blames herself for encouraging the contact int he first place when he didn't get in touch, but I don't blame her, she honestly thought it was the best thing to do because children need their fathers, but I would like it if anyone in this situation could learn from it and save their children years of heartache.
I would like to add thought that it is really good that your DD has a loving step father. I did too and I don't know how i would have got through many things without him. He was, and still is, my rock where father figures are concerned (he saved the day when my father caused a scene and walked out of my wedding before the speeches!!).
At the end of the day your DD obviously has a loving, caring family with you, her step father and baby sister, and that's what's important. She can see what a proper family should be like. Whether her father should become a part of her life only you can decide.