Do not let yourself be bullied any more by this woman.
I bought a copy to support him.
Your choices are to have your son think his father is a criminal pervert, or realize just how hot Mom was when she was 14 years old just like him! She or he will help guide your through this thicket and think through various courses of action.
On the other hand, I feel like he needs to know that he needs to improve on a few things.
Sit with your sister and tell her you are going to research grief counselors and get some referrals.
The idea of her touching him in any way that would cause him harm brings me into such a rage.