I'll tell you what the family secret is...A psychopath was the mother of their only grandchild. Casey ran that household, especially after Caylee was born. She used that child to hold over their heads. They walked on eggshells. They were afraid to say anything much or Casey may leave and take Caylee with her.
ITA all of your description of the family dynamics once Caylee was born. When Cindy said to Casey she had already started looking into custody of Caylee, Casey kind of freaked b/c there was her meal-ticket...w/o custody of her child, she could be asked to leave her family home. Sure she tried to get AWAY from the house and Caylee, to sleep with *fill in the blank*, party at Fusian and any other place she could find. But to actually be kicked out, she couldn't deal with that. That would mean getting a REAL job and supporting herself. They could even take her car from her.
IMO Lee enjoyed a few short years of "only child" status. When Casey was born, she immed. became the apple of her father's eye and was someone for Cindy to dress up, show off, etc. Lee was kinda pushed to the back burner and Princess Casey enjoyed a life of no responsibilities, incessant doting, etc.
The flagellation was probably so harmful to Lee as a child he would have probably benefitted therapy. But, his feelings went unnoticed. Because CA and GA were not willing to admit there was any problem at all, and since Lee was on Back Burner status, they might not even have noticed.
Casey grows into a consummate liar, and that too is ignored by CA and GA.
Why I don't know, perhaps because they themselves were liars. So instead of addressing the problem, they covered for Casey's lies and weren't willing to accept SHE needed therapy- BIG TIME- too. Did Cindy squash this feeling b/c she was so into the "perfect family" on the outside that it just didn't fit into HER fantasy?