Hi B.,
I was in your daughter's situation many years ago. I was 12 years old, an only child and it was just me & my mom. I remember feeling mad, sad, scared, excited, guilty, all of it. My mom tried to include me in everything, (attending appointments, decorating the room, names, etc..). I have also worked with adolescents and it is such a difficult and confusing age. My mom would give me choices to be involved so that I would feel a little more in control. She also made sure to talk to me and would often create times that would make me have to talk or at least listen to her, (walks, dinner just the 2 of us, night-time, letters, etc.) When you do talk make sure you really listen, even if it seems petty, because it may seem like something minor but to your daughter it may well be a huge deal. Also, emphasize the things that you & your daughter can & still will do even after the baby is born, if you don't already have a hobby that is just for the two of you come up with one. Reassure her, hug her, love her and just when you think you have done it enough do it one more time.
My mom gave me a "Big Sisters Survival Box" as a gift from my brother. She wrote cute notes with each gift.It had a bunch of things in it, "headphones to block out the crying, a pack of gum, because no matter what families stick together, a disposable camera, "to capture his embarrassing moments and use it against him," a journal to let it all out, good and bad. A "coupon" good for 1 hour of "mommy" time", all kinds of cute little things. It meant a lot to me, I actually still have the box she decorated.
I am sorry this is so long, but one last thing, my brother who is now 22 years old (and in his last year of college) is one of my most favorite people. I adore, spoil, and admire him. You are going to do great. Good Luck!