Hi! I believe I can be of help with this issue.
First, let me say I can imagine how frustrating and distressing this situation must be for you. Your sister is being manipulated and even blackmailed.
And this is actually the key to my answer to you that you need to consider and think about. She is very fortunate to have you as a sister. You are clearly caring and concerned and she needs a more objective viewpoint because she has lost a healthy perspective on this. So good for you. I urge you to print out my answer and send it to her so that the two of you can discuss it and use it to help her that way.
She has allowed herself to be blackmailed by a user. Is her son in law a full blown narcissist? Very possible. From the behavior of the daughter, his wife, it is very probable that he is narcissistic. He has isolated her from her mother and family and she is completely lost in his orbit. And he is a user even if he is not fully narcissistic. By user I mean that he uses people for his needs, desires, purposes. He does not seek a two-way relationship, a give and take way of relating. Given that he is probably suffering from Narcissistic Personality Disorder (NPD) it is likely that he does not have the ABILITY to have a two-way relationship. People with NPD are very charming at first (courting the daughter). Then when she becomes his wife, he begins to make her personality a subsidiary of his. This is part of NPD.
So, until she is willing to step back and recognize that she has lost her independent thinking and judgment making, she will not be reachable. There is no way for her mother to convince her that this is bizarre and unhealthy behavior to have with family UNTIL she is ready to think independently. This is your sister's first job. To get her daughter to start thinking independently.
The first step is to STOP being blackmailed. You know what the FBI says to victims: if you pay once, the blackmailer will just come back for more and more. It is fantasy to think he will agree to whatever terms he says he wants. He will just stick to those terms until he judges it safe to come back for more. The $20,000 is now big fish. He will not stop with that. And if she pays it, she will be condemning her daughter to more years of subservience by what she is modeling to her. What do I mean?
If your sister pays, she is signaling that this is normal, okay behavior. If her daughter is struggling within herself to have some moral compass, what will that tell her? That he's right!! Your sister MUST begin to model normal, healthy behavior. That is her only chance.
This means she must communicate only through the daughter. And she must say continually to the daughter this is bizarre, narcissistic, using, behavior. Words like that. That it is unhealthy, unnatural. Etc. She has to begin to plant the seed of normalcy back in her daughter. And do it slowly and without wavering. Because long term this is the only chance she has of getting her daughter away from this man. And that is the only chance she has of having any relationship with the grandchildren that is healthy.
I know this was hard reading and it will be a hard discussion for the two of you. But you must have it. And your sister must be firm and consistent. Her daughter is not just with someone who has a quirk or eccentricity. He is a user and most likely very ill in a controlling and destructive way. So, again, good for you for making the effort to help.
I wish you the very best!
Please remember to click the green accept button. Feel free to continue the discussion; my goal is to get you the best answers possible. Bonuses are always appreciated! If I can be of further help with any issue, just put "for Dr. Mark" in the front of your new question, and I'll be the one to answer it. All the best, ***** *****