I haven't read your other responses so I hope this is not repeated information. Here's what we did, I hope it helps. We had the same thing - one on the way while another was still using the bed.
We had the crib that transitioned into a toddler bed, so we put it on the lowest height and got a railing too. This helped her still feel secure with something on all 4 sides, but was safer for her.
A few months before the baby was due, we bought a big girl bedroom set for her, with a full size bed. We bought railings for both sides so she didn't feel like she was going to fall out. After a few months of the railings she didn't need them anymore and we took them off. She hasn't fallen out of bed once.
A couple things we kept in mind. We did not want to have our daughter feel like she was giving her bed to her brother, so we made sure get her in her own big-girl bed in plenty of time she her new bed was comfy and cozy long before the baby took over the crib.
One other thing is to make sure the room is completely baby-proofed. We anchored the furniture to the walls, took out any thing at all that may harm her (choke hazards, diaper changing creams etc, breakables etc). We also put a door-knob cover on the knob so she couldn't get out and roam the house while were were sleeping. I also kept some things in there to entertain her if she was up and we were not (books, stuffed animals) - nothing that would be exciting enough to keep her from sleeping, that was safe, and would entertain her.
I hope this helps. Good luck!