I think your step daughter must feel uncomfortable that she will have a brother or sister soon. Don't get mad of her. Now, she even need more love and concern. Don't give up on her. She will feel it.
Keep up the good work, pretty sure she feels your love, just does not know how to express her feeling yet.
Suggest you listen to 爸爸媽媽坐下來 (FM997) on every Sunday 11-1pm, really great program for all parents, old recording here http://www.997metroshowbiz.com/bama/index.html