Ten years ago, I experienced a stillbirth. I had a 5 year old and a 2 year old at that time. The 2 year old was too young to really get it, but the five year old wasn't. He basically stopped talking for about a year. Eventually, we struck upon the idea of letting him "draw his feelings." The artwork he did was, uhmmm, interesting. It reflected feelings of hurt, anger, fear, powerlessness. Over time they became less and less like that. By the time he was about 11 years old, he stopped needing to draw to express himself. It was a wonderful therapy for the time when he couldn't find words for himself.
That idea came from a friend of mine who is a Birthing From Within childbirth teacher, a method that uses art to identify and release negative emotions mothers and their partners may have about labor and birth so that their labors can go more smoothly. If it can work so well for women preparing for childbirth, I decided to give it a go with my child. It was like magic with him. He could talk about the art and say things that were too painful for him to say otherwise.
Your daughter is a bit younger, but this may be an approach that could help. You could ask her to draw pictures of the baby or pictures of herself being another animal. After she's drawn it, ask her to tell you about her picture and what's happening in it. Ask her how the people/animals in the picture feel.
If she's not into drawing, maybe you could join her make believe and and have her direct your actions. Just listen to what she's expressing and you might be able to read her a story or make up on to tell her to address whatever feelings she's having. Possibilities may include reassuring her that the new baby is doing fine, that *she* is fine, that you are okay, etc. It may take some practice to not send her the message that she "shouldn't" feel a certain way. However she feels is how she feels and she needs to have those feelings respected.
Other things that may help, too, would be to take her to prenatal visits and let her hear the heartbeat of the new baby, or to an ultrasound to see the baby, etc. I am a midwife and routinely let worried older siblings help by holding the doppler probe while I listen to heart tones. It gives them a sense of participation and seems to allay the fear that the grown-ups are not telling them something important. Maybe she could help you do kick counts in the evening... You know her best and will know what might work best for her.