My thing about giving her a doll is that she already has one, she's pretty rough with it and when she gets happy and excited she smacks the baby doll in the face! I'm not sure if giving her another will work.
My daughter will be 18 months and I have been so nervous about how she is going to handle the new baby. We bought her a doll baby and the accessories (a swing, stroller, crib, highchair,etc) so she can be just like mommy with her doll. We plan to set up her doll's accessories next to the new baby's matching accessory so while he is in the swing, she can put her doll in the swing too while we play :) I am just up in the air about waiting to give it to her after he is born or give it all to her now to prepare her...
I've read somewhere...that you can a present for you child and say that its coming from the new born baby.....
I'm due April 1st and my daughter will be just over 18 months I'm worried about that too. We plan on buying her a soft doll so she has a baby of her own. Hopefully it will help. Good luck!