S., I dont know if at 5 months I would let her cry it out. At 5 months you are still dealing with the needs over wants. I have two children as well whos rooms are side by side (the wall is a thin as paper...no sound barrier at all. I just started bedtime a little earlier to get through it. I allowed my 7 year old to stay up a little later and started the routine for the baby a little earlier. At four months I started to keep a rather strict routine it went food then read and play then bath then bottle then bed. I never varied and regardless of if she was asleep or not she still went in her crib at first it was a few min. before she went to sleep and now she sleeps through the night and I have no problems. Babies are creatures of habit and those habits are learned.
5 months is still pretty small though they dont cry for attention at 5 months they cry for need. And it may even be fear. just your presence rubbing her back and standing next to the crib may help her. Your other daughter will get used to the noises of the baby as did my son. She cries at 7am because shes hungry and on the weekends my son sleeps right through it.(during the week he is already up.) If it affects the older childs sleep too much maybe you could place the baby's crib in your bedroom until a routine is set up?
Just some thoughts...good luck...sleeping through the night is the wonder of the world...and its like hitting the lottery when it finally hapens!!
also there is a great book you can get it at any book store called the Baby Whisperer. It deals with bed time. I found it very helpful for both of my children.
I just want to say again I dont think that letting an infant cry is a very good idea. I stress that routine even at 5 months is so very important!!