Being a step-parent is a huge resposibility and undertaking. There are many pitfalls and it takes a lot of effort to make things work. Two of my nephews were brought up with a stepfather, not entirely successfully. My '
Brother in Law clearly favoured his own two children over his stepchildren and as an outsider, I can see huge differences in character between the two pairs of children. Of course there are other factors involved and I don't see everything that goes on, but in some ways his influence is very apparent. That's not to say that there is anything wrong with my older nephews, on the contrary, to me they seem more respectful and less 'arrogant' than their younger siblings.
I also found myself in the situation of being a stepparent temporarily. I found it very difficult indeed. Not being the biological parent, I felt that I had no rights to discipline nor guide and tried to remain detached from those aspects of the role. But when a child looks to you as the 'supervising adult', 'stepmother', etc., it makes it very difficult. There was a definite tendancy for me to try to be more of a friend than a parent, but sometimes the responsibility was unavoidable. When I gave my permission for the kids to do something that they wanted to do (usually only in the absence of their father) things were great. However, when I asked/told them not to do things that were wrong/dangerous/inconsiderate, the resentment started.
I wasn't a perfect stepmom, I never had children of my own and have never wanted to have children of my own. Unfortunately and with hindsight, that may have implied to those kids that I didn't want THEM, which really wasn't the case, I just wanted everyone to know that it was all new to me and that it might take a LOT of practice to get it right. I did the best that I could in the unusual and imperfect circumstances that were given, and where I made mistakes I have to live with that and learn from it.
I have a lot of respect for Step-parents, and Foster Parents, it's really not an easy job. Kudos to those who do it successfully.