Wow!!
*borrows the monster wall of ugly ultimatums and throws all the bricks for Jen's messed up situation*
All this is so screwed up, Jen. Im sorry.
I think the gf gets that the only part of Stephen's life she isnt a part of, is the one which is connected to Anna and you. And this is her way to invade that space as well. I doubt she just wants to be more friendly with you, cuz she's making very obvious attempts to be a part of Anna's activities, which she obviously has nothing to do with.
Isnt there anyway that you can tell her that you appreciate all these new efforts to reach out to you and be more cordial, but you'd like if she drew a line somehwere, and not poke her nose too much in matters that dont concern her at all.
Maybe she thinks that you've accepted that she and Stephen are together and so is trying to make things less awkward for everyone. The trouble is that you cant actually tell her what you feel about this, without sounding mean and rude.