My house only has 2 bedrooms, so when my 2nd was born it was a huge worry for me how to get them in the same room together. Once the youngest was sleeping through the night well, we moved her in with her sister and a baby monitor and it really was not a big deal. Once the youngest got older and they were goofing around keeping each other up (2.5 and 5 years old), then I made a little room in the basement for the oldest, since she was complaining that her sister kept her awake, and things have calmed down a lot. Its not the greatest set up, but the oldest enjoys having her own space.
I'd suggest for you, moving baby in with big sister. They will be going to bed at separate times, so it shouldn't be an issue. If baby goes down first, then have big sister learn to sneak into bed w/o waking up baby - maybe store her PJ's in a different room. If baby has a night feeding, be sure to rock to sleep before putting her back in the room. Amazingly when my kids did not seem to wake up when the other one was awake - even if they were screaming! Kids are resilient, so they usually can figure out a way to make the room sharing work. If I couldn't separate my kids, I'd stagger bedtimes or something to be sure they could get some sleep. I think we as the parents stress about it more then the kids do. :) We just live in a society where we're expected to provide a lot more for our kids then was expected in the past.