I voted that the whole family knows.
After reading through these posts, a thought occurred to me. If Caylee was the result of rape, not only would that explain Casey's detachment towards her, but also would explain Casey's relationship with Cindy. Here's what I'm thinking:
Casey is raped and becomes pregnant as a result. She hides the pregnancy for as long as she can because she is in denial and is terrified of telling her parents. Finally, at 7 months pregnant, she has to fess up to her family. Cindy insists she keep the child and never say anything about how the baby was conceived. They will just fabricate a story for that. Casey tries to pin it on JG, hoping he will fall for it since he's a nice, decent guy but he asks for a DNA test proving he is not.
Imagine the rage Casey might have felt being forced to keep a child that was a result of rape, and then as Caylee grows and Casey acts out more and more, it seems as though Cindy and George are more attached to Caylee and ready to throw Casey out of the house. The thought of this happening puts Casey over the edge and that is when she killed Caylee.