This is I Believe's step-daughter who is 18. My 20 year old step-daughter was not totally overjoyed to find out I was pregnant either. She wanted to remain her daddy's only child. Also, she's pregnant herself so I think she was hoping the excitement would go towards her baby. She is ok with us having a baby now. She loves kids.
I Believe, I really hope your step daughter gets used to all of this soon. I can imagine how stressful this is for you as I know you really want a baby.