That's an interesting idea. But I think also that the us vs. them is constantly changing. It describes very well what I was thinking while I was reading the letter Mary wrote. I was imagining Mary's real interaction with Maria and Julia as being very different from the way she describes it to Fanny.

I'm not sure if I'm right here, but I was imagining she was probably opportunistically playing Maria's and Julia's best friend and putting Fanny down. Whereas in the letter to Fanny she of course has to show her loyalty to Fanny if she wants her on her side.