C.M.
Just keep telling her about her little sis/bro. Get her involved in as much prep for the baby as you can. Let her help make decisions on colors, clothes, etc for the new lil one. Explain to her that she is going to be Mommy's helper once the new baby arrives. All this will make her feel much more involved and hopefully deter some jealousy. My DD was 22 months old when I had my DS and she thought he was "her" baby. She was so proud of him and wanted to be so helpful. She brought me diapers and wipes and would even tell me he when he needed his diaper changed. I got her a baby doll that cried and took a pacifier so she could "mother" it in preparation of "her" real baby. I think since your oldest is a girl that she will just have those mothering instincts. Also, try to have special one on one time with her once the baby comes. Let her know how special she is and that you love her just as much as you always did before the new baby. You just can't beat the bond between siblings. It is so precious to watch them bond when they are so little and it is just instinctual with them. Relax and trust that they will love each other fiercely, cuz they will.
GL with the new one and congrats!