I have been reminded lately of times when the girls were much younger and people would be here for an occasional afternoon and inevitably ask, "Don't you need to go get the one who is crying so she won't wake up the others?" Ha, ha!
We have had all our trio sleeping in the same room together since the day we brought them home from the hospital, 4 days after they were born, and they were together there and in much closer quarters before then. Since those early days, they have simply gotten used to the sounds that their sisters make and can sleep through some of the loudest, most obnoxious screaming you have ever heard.
On Sunday afternoon, after Maggie and Kate fell asleep for their naps, Sarah decided she needed to go to the potty. I collected her over the gate and took her to the bathroom. When she was through, I politely placed her back inside the gate to their room and instructed her to get back to bed. She appeared to comply, and I started to walk off when out of the corner of my eye I looked in to see her standing at the head of Maggie's bed, reaching over her sweet, soundly sleeping sister, and grabbing the plastic headed baby doll and her precious pink lovey that she had no doubt tossed onto Maggie's head when the urge to go first hit her moments earlier.
So, yeah, they can sleep through almost anything.... including the cries of the others.