How is it proven ? Obviously it was not or the judge would have let the DT discuss it during closing.
Let's take the opposite side ... how do we disprove this claim ? Well, look at FCA's videotaped conversation with GA at the jail ... "Dad, you've the best father and grandfather .. you've always been there for me" etc, etc. Also, do you truly think if FCA was molested by GA, she would have left Caylee alone with GA or not confided this with CA and CA would have taken action against GA ?
It was obviously FCA's decision to make up this ridiculous claim against her Dad and partially Lee, as she had nothing on CA to throw her under the bus. If I were George/Lee, I would sue FCA as well. There is not a decent bone in FCA's body.
On a side note, I wonder why FCA/JB cast aside the idea of blaming JG for the abduction/murder of Caylee ?