My belief is that it goes back to the grand jury indictment. I think KC blames George for testifying, and Cindy for not keeping him from testifying.
KC doesn't see it as her own fault she is in jail and in a hot mess because she murdered Caylee. She thinks she can and should get away with it, because her parents always let her get away with everything. I think in her mind, it was up to Cindy and George to somehow keep her from being indicted, despite any and all other evidence and testimony at the GJ. They failed her. She's pizzed. And she's not going to talk to them or 'be nice' to them until they get her out of this mess.
Note that this would also explain Cindy and George's behavior so far as their 'KC is innocent' campaign, which has, to me, remained otherwise inexplicable. This family is very enmeshed - they do not function as individuals, but as a melded single entity. They all have their roles within this entity, they all play their parts, so the group can function as a whole. There's no separating them. When I look at the behavior of one, I now always look at the behavior of all the others, and it's much easier for me to make some sense of what they're doing.
MOO