K.K.
It is so hard to know what the build up on both sides are. I met my 'dad' when I was seven. Things did not go well (met in bar, he complained about my clothes, did not really engage with me but more so my mom). I think you are already on the right path by thinking of her. The main thing I would recommend is prayer. Prayer that God will intercede this reunion and make it what He wants it to be, despite if things are said wrong or if you just end up staring at each other on the couch. Beyond that, relax and be support for your husband. She is not going to call you 'mom' of any sort, that's my guess; I refused even when strongly encouraged. She will likely be nervous as well, not sure of the unspoken expectations such as this issue.
The only advise I can share is more for you alone. My stepmom was jealous of our (me and my twin) relationship with our dad (this dad adopted me at 2). It is ludicrous, but she was and to some extent still is. Allow your husband to be available, with or without you, for his daughter if she so desires. I feel my stepmom had to 'approve' every visitation and it really divided my dad and me for awhile. As an adult now, I deal with it more overtly, so it has gotten better.
I think people tend to relax over a meal, so a nice dinner may be a good start. Especially if there are yummy desserts; what 12 year old girl doesn't like chocolate?
I hope all goes well---it's a big time in everyone's life that is involved.