Cindy and George didn't for 1 minute believe Casey was an angel. The girl was always lying, stealing money, never graduated from high school. Yes, they did their share of enabling Casey. They probably shouldn't have supported her they way they did. Looks like once Caylee came around, they were just plain scared that Casey would up and leave with the little girl.
Im sure they knew that Casey didn't have the means to take care of Caylee. Everyone around them told them that Casey had issues, but sadly, they just tried to cover it up for appearances and for whatever reason, further enabled the behavior. Ive seen parents do much worse.
But even through all these lies, you can see they kept giving Casey chances. Do you really think that they believed, for even 1 minute, that Caylee was missing because Casey killed her? I don't. They were probably more worried about who Casey gave her to! They must feel every emotion in the book. They probably feel overwhelming guilt as well. I don't see them as evil. Misguided maybe, but not evil. They wanted to believe their daughter was good, sadly its hitting them smack in the head and it must be very painful. They need to admit the truth about Casey for the first time and this time, they can't help her out of this mess anymore.
At least, thats how I see it. Today... (reserve the right to change my mind as testimony continues!)