I think it gets more credible if we suppose that she can only hold one set of manners in her head at a time: she's spending her time with Mrs Fraser and Lady Stornaway, so she addresses Fanny the same way she addresses them.

Without deciding the question of whether those ladies led Miss Crawford astray or vice versa, I wonder whether they're in the habit of behaving like a school of fish: whenever one of them makes a dart in a new direction, the others automatically stay in formation with her. "Here, Fanny, I'm heading around the dark side of the coral reef. Keep me company!" Back when the Miss Bertrams were at Mansfield, they were the trend-setting 'alpha' fish, and Miss Crawford was more of a follower fish, such as when she obligingly joined in the glee, for example. As Edmund said in ch11, how readily she falls in with the inclination of others!