comfort her the way you would if you had the twins, yourself... "honey, it's always extra work to have a new baby, and it's worse if there are two. But that doesn't mean your Dad doesn't love you any more. You are his first and that's very important. These babies are going to think you're the greatest girl in the whole world because you're their big sister, and you just have to be thier sister and wait for them to get old enough and you'll see what I mean."
AND THEN... tell her to talk to her Dad and stepmom about it. AND THEN tell her Dad and stepmom that she is feeling a little left out and that you told her to talk to them about it, and tell them what you told her about it, so they can prepare for what to say to her from thier point of view and the adults in this situation have all presented a united front on it.
If you do all this and the ex doesnt' agree or does something jerky in response, then that's his problem, not yours. You'll have done the right thing.
This one was easy for me because I'm a stepmom & I know exactly how I'd like my stepkids' mom to handle stuff like this... I'd LOVE it if she could figure out how to reassure the kids that she's sure we love them, and then point them back to us rather than letting her own venom inspire her in how she handles thier angst.
Her response to something like this would probably be, "well, honey, he doesn't have time to spend with you right now because he's too busy with his new family, but I do have lots of time for you because I love you, and I know the babies are so loud and annoying that it must really hurt you to be there so much. I'll see if I can't convince him to let you stay home with me rather than be forced to go over tehre for visits."
I can see it now. It would SOUND so nice, while giving the poor kid all kinds of wrong ideas.
So I've given an example of the right way and of the wrong way. Please choose one, do it, and let me know which you chose and how it worked out (I'm SO praying for you to choose the one that does good things for the kid rather than the one that does good things for your own ego!)