DP and I got together when DD was almost 2, we moved in together about 9 months ago and now have another DD together. Before moving in we had sleep overs both at his place and mine where I would step back to let him be involved in caring for her needs. We also discussed our parenting ideas early on as DD is the most important thing in my life so wasn't about to waste time with someone whose thoughts didn't align. When we moved in DP just continued to be active in her care and I just let them work out their relationship together with minimal involvement. Now you could never tell by looking that they aren't related, he is equally good with both girls.

So the main thing for me was just giving them space to work it out in their own time. You really do need to trust someone a lot to bring them into your child's life so that you are able to take that back seat. We never had any problems once we began living as a family, I hope things go smoothly for you also.