I don't think they discuss sleeping arrangements, but they were clearly not supposed to be living there, and they each have a room. Obviously the sleeping arrangements could have been discussed and the nuns shown their rooms off camera.
There is another weird line, when Rose says she's never been thru a hurricane, Blanche says she has, and adds, do you remember sister Rose? But sister Rose wasn't there because she just said she's not experienced a storm. Of course she could reminding Rose of a story Blanche told her once about the storm.
Over analysing?