The answer to this question for all of us is very specific to our own experiences.
In MY own personal experience, me, my SO, my ex and his SO, and our three kids live in peace because neither SO parents. Perhaps the ages of the kids make a big difference?
When my SO moved in my kids were 11, 14, and 16. As a family we were already a well oiled machine. If either SO had stepped in and tried to change that, I'd imagine a LOT of problems would have ensued.
Both steps love and support the kids. All kids have a great relationship with both steps, but neither step 'parents'. There is just no need for it.
So I have to take great issue with SH when she says a step who doesn't parent is not a real member of the family (although I love her still).
Goes back to in OUR situation, this is what is working. Might not work in EVERY situation like it does in ours.
:)
**I should also mention there are no other kids involved. No existing stepsiblings and no new stepsiblings. I can certainly see how in the event of 50/50 custody where there are other kids living in either household, this would be a tricky situation. Everybody does what they need to do, no black and white, right or wrong here.