If you have a humidifier try using that in the house to raise the humidity to a comfortable level (if it's good for you it's good for them). Our own skin and nasal passages dry out during the winter especially due to heating.
If you do not have a humidifier (these can be found in the pharmacy section of walmart, etc.) simply put a large pot of water on the stove at a slow simmer...enough to see it giving off steam. Keep a CLOSE eye on it so that it doesn't run dry and I wouldn't suggest leaving it unattended when you aren't home. This is something I learned from my husband's mom. She did this and to add a little holiday cheer she would toss in orange rinds and such.
When the nasal passages dry out and crack...that is what leads the way for sinus/respiratory infection. I would get your humidity level up. Give the cages a good cleaning to rid any possible infectious gunk. Change out all of the pouches and other materials as well, but try using some that have only been soaked in boiling hot water instead of whatever your normal wash routine may be. This way you can kill any germs but not introduce smells in the fabrics that he may be sensitive to at this time. Of course, this will set them all into hyper scent marking mode, but that's better than possibly passing around an illness.
Really, it's all I can think of at this point...if it is related to any of this. I really do feel that it is humidity related. If you've caught it soon enough you may have dodged a bullet. If she gets ANY worse...just try to plead your case to a caring vet. A tiny bit of antibiotic in the beginning stages can make ALL the difference in the world to these tiny critters.