I think that Skakel is the only Kennedy who has ever been punished for his crimes. All the rest have gotten off with “get out of jail free” cards.
I think the reason is that he is so vile that not even the Kennedys can stomach him. And that must be pretty vile indeed, when you look at some of the rest of them. So they stood back and let him be convicted.
Instead of what exactly?
Give this SOB some Preparation H and tell him to get over it! He doesn't have the "Camelot Mystique" cloak of invincibility...
Or .. they knew he was guilty .. and they let him take his punishment!