Totally with you on the stepmother front; I think that title it bestowed too easily and should really only be used when the relationship can realistically be defined in those terms.

So for example, neither of my dad's partners, nor my mother's late husband, have been my stepparents. I am not Michael's don's stepmother, because he does the parenting, but I WAS Darren's daughter's stepmother because I did much of the parenting when she was with us, and that was the sort of relationship I had with her.