Where do I start? I've been with my DP for 10 years. He has 5 kids with 3 different mothers, the older 4 are adults and the youngest in his teens. He had his problems with the first 2 ex's but they are pretty much history now, the latest ex (mother of his youngest son) is another story.
When I first met DP I was impressed with the relationship he had with his children despite the access problems he had when they were younger. Unfortunately he was going through it all again with his youngest - the BM pulled all the strings - limited access on her terms, no overnight stays, no holidays, no Christmas access - and any deviation from these terms on his side resulted in her making access difficult for a while. Then there was the abusive phone calls after each visit accusing him (and me on occasions) of not taking proper care of SS because of some petty thing (he got food down his clothes, he had a bruise on his knee!!!) But SS was still young, DP and his ex had only been divorced a couple of years (although had been separated for 4 years), if we kept our heads down and kept her happy one day SS would grow up and we wouldn't have to have anything to do with her any more.
I didn't take into account DP's family though. A few years into our relationship there were some big family celebrations - a wedding, birthday parties, christening - and the ex was invited to all of them! Far from gradually disappearing into history (with her new husband) she (and him) were becoming more and more included in the family thanks mainly to DP's brother and sil. And then she discovered Facebook and reconnected with all DP's friends that she had once known and began turning up at friend's events. Although we boycotted a few family events and told them the reason, it fell on deaf ears.
Then DP's eldest son announced he was getting married to someone who he hadn't known for long. DP was concerned he was rushing into the marriage and wanted them to wait. A row ensued which I thought would soon be resolved (any disagreements between them before hadn't lasted long) but not this time and to add insult to injury we found out that OSS had gone to the ex (his ex step-mother) and she was helping organise the wedding. We were invited but only as guests, ex and hubby took our place and I was left out of the photos.
It was not long after this that his eldest daughter, who had remained friends with her ex step-mother, decided that she was no longer having anything to do with her dad. She married this year, again the ex helped organise the wedding (she is now friends with 1st ex wife!) and DP was not invited at all.
OSS has now got 2 children and although DP insists that I am 'Nanny', I don't feel as if I can fit into that role while the ex is still involved so much,
I feel as if his whole family still see her as their step-mother, daughter/sister-in-law, aunt etc but where does that leave me? DP tells me it isn't important, it is US as a couple that is important. But then he wants me to be part of his family. I've tried just being 'me' in the hope that they accept me more but her relationship with them just goes from strength to strength.

DP seems to be able to ignore everything his ex and family do but it isn't him that doesn't feel as if doesn't fit it even after all these years. Despite numerous discussions about how I feel the only thing that comes out of it all is that I have got to change my way of thinking. I'm beginning to accept this but I also feel as if all the loyalty and support I have given him (and I have put myself through hell at times) is being thrown in my face. I got dragged into all this, now he backs out and leaves me with all the pent up anger and frustrations. I could have not got involved at the start but I couldn't stand by and watch someone destroy the good relationship he had with his children that attracted me to him in the first place. And how do I suddenly change the way I think? It's took 10years to get to this state but if I don't find a way soon I fear our relationship could suffer.