Becoming a step mom to an newborn.

Me and my fiancé broke up last year for a little (I broke it off) and he got drunk one night, got a girl pregnant, things happen. So don't judge. We are back together and have been together since we were little kids and geeting married soon so, I do love him, and plan on being with him through this. So my question is, since I'm gonna be in this little girls life from (legit) day one...what are my boundries? I'd never try to be a mom to this kid. But do I be a motherly figure? Or just a friend she can talk to when shes older? I don't know how this works. I know when a woman became my step mom when I was a teen and tried telling me what to do, things wasn't pretty. But since I'm gonna be in her life from the time she's a baby to adult, what am I supposed to do? What is MY role?