Lots of people never get married, so who officially gets called "step" would miss out a lot of people if it was strictly followed.
In a study about parents/parental figures killing their children, the author called non-married step parents de facto parents. I guess that would be a good word here.
In terms of Tustin, she ought to have been acting as a stepmother, but she didn't give Arthur the slightest bit of parental care. Her own two children would have suffered second-hand seeing him suffer, and seeing their mother and Hughes behaving like that. It would have been very confusing and emotionally painful for them. Now, they've lost the only mother they've ever known. They might end up with survivor's guilt when they get older, too.