I have never posted on a step-parenting thread before (they usually turn into a bun fight and I can't deal with the emotions that stirs up) - but I was brought up by my mum and SD, was close to my SM, and have been a SM to 3. No children of my own.
Right from the beginning of my relationship with DH 19 years ago, I saw my role in the children's lives as - virtually nothing at all. My job when they were with us was to be kind, generous and easy going, and as long as everyone got fed, nobody died and everyone was courteous to one another, that was good enough for me. If discipline was required, DH took over (unless he wasn't around, which happened rarely) - I was capable of breaking up a fight, soothing a bump or a scrape or stopping them from wandering into danger or whatever, but I wouldn't have dreamed of commenting on anything else. I saw myself as being entirely non-influential - I had no wish to upset or undermine their mother, who was a nightmare for many reasons and is much worse now they are adults but who was doing the best she could as a single mum, and I didn't want the kids to view me as anything other than a quiet, benign presence really.
In my mind the children and their relationship with their father came first - I accepted early on that I would ALWAYS be second, when it came to anything that involved them. Whatever he wanted to do or arrange in terms of contact, I acquiesced with no question. I was petrified that one or more would want to live with us at some point in their life (I just couldn't see myself coping), but when it actually happened and DH agreed to it without really consulting me, I accepted it immediately even though I was dying inside because, how could I object? It wasn't about me, or DH, or the ex - it was only about them and the relationship, and that trumped EVERYTHING including my own feelings.
The only way I could dismiss myself so easily was knowing that it wasn't forever. The kids would grow up and make their own way and all this would be a distant memory.
And that's where we are now. They are adults, I care very much for them all, we see them all regularly and do stuff together, they are kind and respectful and all doing well for themselves.
However - I do have one big regret, and that is that in removing myself from any position of influence, to preserve their relationship with their father for all those years, I also effectively removed myself emotionally. I didn't touch them or show physical affection when they were little, and that gulf is really showing now that they are adults. I am a little afraid to attempt to give an affectionate kiss or hug hello/goodbye, so I hold back.
Only a couple of weeks ago my SD voluntarily hugged ME. I think the last time I touched her was 10 years ago. I was quite overwhelmed, as though a barrier had been lifted. I don't kid myself that they are in any way fond of me, but I love them, I hope they like me and that's enough.
I'm not sure if any of this is helpful to anyone, least of all the OP - you have a child with your DP, that changes the dynamic entirely and I haven't been there - but ultimately, I got through the step-parenting maze by stepping right back.
They may think me a cold fish, 20 years on. But ultimately their relationship with DH was more important than mine, and I am safe in the knowledge that I never stood in between them.