Personally, I just don't see how Fredo could've opened the drapes, and not know it would be a hit. What did he think? They wanted photos of Kay in her underwear? And I do believe that he did NOT set up his brother for murder. That is evident in his late night conversation with Johnny Ola.

I don't have a clue who did actually open the drapes, but I have problems believing it was Fredo, mostly because I do want to think of him as stupid, weak and sweet.