I also think it's the adults and not the kids, that can frame the relationship between step parent and child.
I have such a good relationship with my step children and we are now at a stage where the 2 families have merged, the 2 sets of kids all get on and it's turned out so well.
Thing is, that didn't just ' happen'. My sc's mum entered a relationship around the same time, and they have kicked back a bit more on that side and it's a bit more forced. That's not to say her new dh isn't a lovely chap, he is, but they handled the new relationship a bit different and the kids have reacted to that and it's grown from there.
Time and respect for space, is probably what I'd contribute to success. Also the fact that all the adults involved have made a conscious effort to get on and have an, at least, cordial relationship where we can all meet for drinks or children's events, and it be a positive experience, although to be fair, this is the worst bit for me the run up always makes me a bit anxious, but is actually quite enjoyable, at the time. I've ended up actually quite liking the ex wife and her husband...
And don't get me wrong, I've come on this thread all, like, Disney-smug, but it didn't come without work and struggles and frustration. I'm a decade down the line now, but I can't emphasis how bloody lovely it is when it does work. The feeling when you watch your kids, your step kids all in a room laughing and joking with each other, organising days or nights out, independent of the adults, is second to none.