Even if a DC lives with and is raised by a step-parent, that step-parent doesn't replace the absent or less involved parent, and the goal of both the parent and the step-parent should be to ensure that both the child's parents play as significant a role as possible in the DC's life.
Yes...may I add to this? My DSCs' mother is deceased, so I do "parent" them - I have cooked and cleaned for them, given advice on friendships, made fancy dress costumes for school events, kissed away tears, punished them, praised them, taken for first bra fittings, had 'sex talks', helped with coursework, and so on. My number one job, as far as they are concerned, is to help my DH make sure they are safe and provided for. My number two job is to make sure they feel as secure as possible in our family, and that means making sure their mum is always respected (even though I disagree with a great many things she said or did!). In my situation, it is probably easier to see how much that matters, frankly, because she is dead, and it is not the done thing to compete with or hold grudges against a dead woman. But I don't think it is all that different, either, from how one should treat the living mother of a stepchild. No matter how vile she is.
I obviously actually think it is perfectly okay for a step-parent to take on parenting duties in the absence (whether temporary or permanent!) of the mother or father - but one should not in most circumstances confuse that with being a parent...and when/if the absence ends, the step-parent may well need to step aside a bit. More importantly, even during the absence, the step-parent should prioritize the role of the mother and father. That doesn't take anything away from the role of stepmother, which has gotten way too bad a rap as a label, and ought to be seen as a very special relationship on its own merits. One's stepkids may end up seeing one "as a parent", but it seems to me that is something that can only develop over a very long time and expressed in adulthood, and certainly not is something that can be engineered by the step-parent.