Wow, Jen...I'm sorry for your crazy situaution! *throws brick*
I think Veronica made a good point - she probably sees you and Stephen's relationship with Anna the only thing she's not a part of, so she's trying to edge her way in. If that's the case, and you're not completely comfortable with that and the way she's going about it, I wouild definitely have a talk with her. Tell her that you understand that she wants to get along with Anna, but that you feel that she's over-stepping her boundaries. In as nice a way as possible, of course. That's only a suggestion, though. God knows it's easier said than done.
Best of luck Jen!!