There is no way that Vito or Sonny would've ordered Fredo's death. They wouldn't have been able to do it. Witness Sonny's utterance to Carlo, Touch my sister again and I'll kill ya (one of DC's favorite lines, btw). If he had any real intention of murdering Carlo, he would never have uttered that phrase in front of witnesses. Vito and Sonny were more "old school" and I don't think that they could've ordered the death of their son or brother.

Of course, if Sonny and Vito were alive, then Michael may not have been the Don, but that's a whole 'nother nut to crack.

And of course, Fredo knew that he was going to be murdered.