I think it depends a lot on the context. I can only speak of this subject from the point of view, in my case, of the daughter. In my case,my father died when I was very young and my mother remarried, which I think was a good thing. The only thing which I found very negative and that took a toll on my relationship with my step parent and my mother was that they expected me to treat the step parent as my father and that had the reverse effect, because I resented it. Also, resented the fact that my actual father was sort of forgotten about. So, I think that this sort of question is very delicate and it has to do with the context that led to it and also how the son/daughter feels, it's very important to talk to them and understand how they feel and why. But the main thing, for me at least, is to make sure the child understands that that person is not replacing the other one, that there is still room for the other parent, because really no matter what, it is still her/his parent and that bond (apart from a few exceptions is very tight and if one tries to break it, it will probably backlash. Given this, I would say the main difficulties are steping into the parenting shoes, without the shoes actually fitting and trying to balance the responsability and authority necessary with the fact that there is or there was a parent and that it's not possible to put that parent's position into question. But anyway, this is just my point of view, based on my experience... All the best for you and your family.