I am not a stepparent, however I do have a dp and my ex has a dp who also has a dd.
Tbh, I find this idea that the stepparent shouldn't be involved in parenting in any way utterly bizarre, and even more so when they have children together. How can it be possible to have a harmonious household when each parent parents their own children and both then parent the joint children?
While I do think that the enforcement of the discipline needs to come from the parent, I do also think that in order for a new relationship to work everyone needs to be on board with the rules of the house, and that everyone should be in a position to enforce them.
When you get together with someone who has children there does need to be an acceptance that those children come as part of the relationship, and that as stepparent you will have some responsibility towards those children.
IMO a lot of the issues with stepparents come from the fact that they resent the dsc, feel that they have somehow encroached on their relationship, and that they are somehow responsible for the wrongs which happen within that relationship. I have even seen posts on here from stepparents who feel that they shouldn't have to take their sc to their family's house because that should be the preserve of their own children.
And ime the majority of this resentment and bad feeling comes from women. Personally I think that this is because women are far less tolerant, so if a man has issue with her kids she is more likely to get rid of him sooner, whereas for a man if the woman has issue with his kids he is more likely to bury his head in the sand and allow the relationship to continue regardless, esp if he is nrp and only sees the dc every other weekend so the issues only manifest during those times, iyswim.