What I will write is from observations of the same thing. My mother died when I was 19, and my father married a narcissist two years later, soon after I moved abroad. Fortunately, this did not affect me emotionally for two reasons: 1) he was a strong man and divorced her within a year, and 2) I only spent one summer (3 months) in the house when she lived there. She destroyed him financially in the divorce, and he never recovered.
But the dynamics, based on that and two other cases, and reflection... Most men, when widowed, get insanely lonely and desperate. That is the perfect victim for the N. They undboubtedly provide the person with something: a promise of security, great sex, affection, etc. that the widowder desperately needs. It is often possible that this N has provided something that the previous wife was sorely lacking also, especially passion. Another factor is that after being married for a good number of years, the person is suddenly alone and has no idea how to "date." In most cases, they have forgotten what to expect from a new relationship and how to proceed. They have been used to the married life. So they probably do not know how to deal with a new personality.
Finally, a reinforcement pattern sets in. (Excuse me, but I am a behaviorist and see these patterns.) Typically the paradigm of "negative reinforcement" sets in. Most people think of that as punishment, but it is different. It is an increase in the behavior to avoid some pain. Pinch an arm, and tell the child you will stop as soon as he lets go of the rabbit's ears, for example. Punishment is pinching him after he drops it. Anyway, I see so much of this. One spouse yells and nags, and the other finally "gives in" to stop listening to the ranting. They will say that they had no choice, but what they did was reinforce that nagging behavior. It will become worse the next time. After years of this pattern with many behaviors, it becomes chronic and rarely can be stopped. Your stepmother probably knows how to get what she wants. She is probably the talented manipulator who can press the buttons and get the candy.
I do not think you should go into all sorts of psychodynamics and history. It leads nowhere. It usually is just an exercise of egotism. Look at the behaviors themselves, in the present, and don't reinforce behaviors or thoughts that will lead you to suffer.
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