Well, I did mention that we have no children of our own - I never wanted any, and he had his very young (he was just 20 when his first was born) and didn't want any more.
I think things would have been very different had we had a child of our own - as I said the dynamic would be totally different, as I would have fought harder for their "rights" and position. There is compromise in there somewhere, but it would have been extremely difficult and I don't envy the OP.
I myself was the youngest of 7 step-siblings, 3 from my father's new marriage and 4 from my mother's new marriage. I never knew any different, everything was normal to me, but I didn't notice the imbalances until I was over 30! That's a whole new thread...but from experience, I can say that children are remarkably adaptable and many of the emotional traumas they suffer as blended families are emotions projected by the adults. IMHO.
Anyway, yes there was a degree of self sacrifice, but knowing how difficult my relationship is with my father, I didn't want that for any of them, or for DH. I was prepared to step back to spare all of them that - they have still suffered it in some ways, no parental split is without drama or trauma of course - but I can sleep at night knowing that none of that has anything to do with me. The ex has tried her best to implicate me and alienate me, but there is nothing and she failed.
I don't believe I have paid any price. Mine and DH's relationship has never wavered, we never argued about the children (we fight all the time over the bloody dog) because I simply gave way.
I guess I am fortunate that DH is not abusive (meaning he never took my passivity for granted or abused it), and the children were always calm, placid and respectful. Awful as she can be at times, their mother must take some credit for that (and DH of course). I very rarely felt any resentment - I'm not saying I didn't cry quietly at night a few times, or bite my tongue, but I opted for silent acceptance.
I don't see that as a life half lived. We're in our mid 40s, the children are grown, and we have the rest of our lives to bicker over trivial things without affecting any children. Happy days.