Thanks so much Janet and LordKAT for helping me out with this.
We've talked about it a little, and I think I'm gonna use female pronouns and so forth when I'm talking to her mother. Wren has started sort of cringing when someone calls her by her old name, so they're going to have to get used to it sooner rather than later.
With the whole family I'm going to stay in pronoun avoidance mode, because I'm pretty sure that no one's told her little brothers yet (her dad wants to "shelter them" because he doesn't want them to get corrupted by her. they're that kind of christian >.<) and they should find out from her, not from me by accident at the dinner table. I actually kind of hope that they haven't been told yet, because no doubt her dad would skew the whole thing sideways.
I don't know that I'll bring printed materials with me, but I'm definitely collecting useful online resources to point her mom to. She really does want to do what's best for Wren, and I think once she understands the whole thing better and sees how much happier she is she'll come around. The main thing I'm worried about is her being dismissive, as she's convinced that it's just a phase that will go away if she ignores it long enough. She compares it to her smoking pot in college, and how at the time she thought it was the right "life choice" for her.
Anybody have suggestions for mom-helping resources? I'm pointing her here first off, as I think this is a good place for her to put some basic fears to rest and such. Possibly questioning transphobia as well? She's so deeply entrenched in the gender binary (doesn't even believe in gay people) that I don't even know where to start