If you go to My Favorite WEB Search Engine and enter in +"humidistat", you'll get a ton of hits on Humidistats. One site was Sensaphone Humidistat. For $40.00, you can buy one to check the humidity in a room. At places like The Sharper Image or maybe Radio Shack or Edmunds Scientific or more, you might find one cheaper or even one of those types that has a Barometer, Humidistat, and a Thermometer all in one! BUT, for the moment, don't run out and buy one until you've tried the next test.
If you're not already doing this, open the shears a little to allow the heat and moisture out from BEHIND your shears. In other words, if your shears are not allowing the air and moisture out from behind them, then it will 'collect' there and condense. Open the top and bottoms a little, especially, and the sides too if possible. You might want to consider 'tying open' the individual shears with a lace band. I don't know how to easily describe the latter but in our bedroom, if we unhooked the lace strap at one end, our shears will 'lay/fall' in front of the window. We don't have a bay window in our bedroom; just plain double-hung windows. So, the 2 shears fall and meet in the middle where the windows (next to each other) meet.
Now, what you might want to try as a last resort is a different material for the shear (temporarily). What I mean is take down (and clean if necessary) your good shears and put up a different material to see what happens. If you get the same problem, then you know it's not the material being overly 'collective' of moisture. As for fabric treatments, that's out of my league. So I'm thinking that the moisture is being trapped between the bay and the shears/curtains and not able to get out. Create some NATURAL circulation at the top, bottom, and sides, and see what happens. Heck, for test purposes, if you remove the curtains and shears, does condensation still form on the window? If not, you have your answer (it's circulation!) If it does, water is coming up from somewhere.