Narcissism is the absolute right diagnosis here. My husband's family invented it, I think! You know, the problem w/ people like your step daughter is that, because they are narcissists, it's as though you are Maxwell Smart (remember that show) and you are in the cone of silence....your lips are moving but she doesn't hear you. My suggestion is that you let your husband work out the relationship as he sees fit and you go on with your love and your life w/ him.
I've learned, at least most of the time, to ignore my in-laws all together. I only remember their existence when they call....as always...needing or wanting something. Maybe narcissism is contagious?