Colic with one was very challenging for me but I admire you and your DH for doing it with two. Don't worry about your older boys. DS1 would get himself up in the am (as I had only fallen asleep around 6 am since little bugger was up ALL night screaming), get his bowl of cereal and turn on the TV. I would get up around 11 (The Price is Right!) and try to spend some time with him. He was so understanding.
Do you have friends that can spend time with the boys? That was the most amazing thing for me. We had a few friends that would pick DS1 up to play, swim, or just get out of the house since I couldn't ever get out. I understand your thoughts about the sunlight. I planned for a baby in the summer because I thought that it would be so nice for DS1 and we would get to do more. He has already asked that if we have another baby I have it in the winter because you stay inside in the winter anyway. He will tell me that the summer that DS2 was born was "awful!"
Good luck and vent as much as you need. It is so frustrating but "this too shall pass." Keep those babies swaddled, moving and run your vacuum all of the time.