I'm slow on the uptake, yes, but now I see that there are too few adult bees to keep two frames of brood warm enough. That is probably why there is so little brood?
Will reduce space with insulating material and add one modest frame of brood from a strong hive and another when those have emerged.
Maybe that's a good plan?
Adding brood adds more work. As you described the situation, you have bees that already have too much work. So ... Do not add brood! Instead, add bees.
There are two methods of adding bees to a weak hive from a strong hive or group of strong hives.
Option 1. Shake them some bees
1. Put a ramp board or cloth to the entrance of this weak nuc. Something that bees can walk up and walk in on.
2. Next go into the heart of your strong hive(s). Into middle of their brood nest. That is where the nurse bees will be. Pull a frame full of bees. Make sure their queen is not on that frame.
3. Take the frame of bees over to the nuc and shake the bees off as close to the entrance as possible, onto that mini ramp you made. Give the frame a single very firm very hard shake. Go put the frame back in the mother hive.
Repeat 2 and 3 as many times as necessary based on the following formula:
3 frames of bees is required to look after 1 frame of brood. If you have 2 frames of brood in the nuc, shake 6 frames of bees out front. If you have 3 frames of brood in the nuc, shake 9 frames out front.
The young nurse bees will walk right in and join the household forces of the weak hive. The older bees will take flight and go back to their mother hive. So do not be shy about shaking lots of extra bees. Take as many frames as you need to from different hives to do the shakes. No worries. Nurse bees mingle and join up together just fine with no fuss or fights. It is the older bees that fight. By shaking out front, those older bees will just fly off so there are no fights to be had. The nurse bees are too young to fly, they will just march up the ramp together and go right into the hive.
Option 2. Exchange hive positions
Move the weak hive to the location of a strong hive and move the strong hive to the location of the weak hive. When the bees take flight going about their daily beeziness, bees leaving the strong hive will return to their old location, which is now the weak hive. Thus boosting the population of the weak hive.
Give that weak hive bees by method 1 or 2 above. Do not give it brood. It needs more workers, not more work.
PS: If you do not see a very significant pickup in the amount of brood being made and health of that brood within 10 days after adding a bunch of bees, then the problem is the queen in there is a dud and needs to be offed by the hive tool.
Hope that helps!
Good luck with it.