Hi! I believe I can be of help with this issue.
First, let me say it sounds like you've put up with a lot for a very long time. And it sounds like you've given in so many times now over these 20 years that your husband does not feel any need to try to back you up in the struggle with his daughter.
On the other hand, you understand that she's narcissistic. And narcissists do NOT give the people in their lives the opportunities to back each other up against her in any normal manner where they can tell her she's wrong this time and has to give in. For a narcissist, it's always victory or the world will end. So your husband is in a tough spot as she does not leave room for any maneuvering.
And this is actually the key to my answer to you that you need to consider and think about. Your question is really in essence how to help her become more normal. Because if she were more normal, then you could tell her she's being too manipulative and she'd get it; her father could tell her no and she'd understand. But you know from your years of experience that she will not accept help let alone be normal. Why not?
Because narcissists never believe the problem is in them. The problem is always in YOU. So if they ever seek help, it is only to get what they want. Then they stop coming to therapy once they've convinced the people who have forced them to that they have tried. And all the while, they keep doing what they believe they should be doing.
I'm concerned more about you. What do I mean?
Your goodness is not going to one day make her see the beauty of also being a good person. Narcissists don't change like that. It takes years and years of therapy and very few ever commit to that. And your husband doesn't know how to change that dynamic and you know that in the end it will be a choice of her or you. With narcissists it always is.
You're a good person and I can understand: you've invested a lot into this relationship and you may decide at this stage in your life that the stability of your life is worth more than the upheaval of change. I think these are the parameters for your making the decision. You love your husband. But to stay means accepting this dynamic for the rest of your life because it is not going to change. I'm so sorry that this is the nature of her illness.
To stay sane you have to accept that she has an illness. A real illness. And a serious one. And this is how that illness is tolerated by family members:
Step 1. You must accept that you cannot change her. This is the heart of the matter: what you see is what you are going to get for a long time (see step 2) if not for the rest of her life. Your job and your goal is to learn how to accept her the way she is and not be affected negatively by her. Yes, this is a tough, big job. But that is what she needs. You cannot be her therapist; you have to accept that this is how she deals with the world and that your job is to be there for her and with her without you getting too hurt by her personality difficulties.
Step 2. You can encourage her to seek professional help. Again, this will have to be only an encouragement. You know better than anyone that even encouraging her will probably not go over well with her, but that is the most you can do in that area. She has to be willing to go to therapy to help herself. No one else will ever be able to get her to stay in therapy and get benefit from it even if they convince her. The chances of her thinking this is a good idea are very slim. Therefore, you go back to Step 1.
Here are a couple of excellent books you can get on how to live with a narcissist.
1. The Wizard of Oz and Other Narcissists: Coping with the One-Way Relationship by Eleanor Payson. This is a great book that will help you with the lack of self-esteem that living with a narcissist or being close with a narcissist will do to you.
2. The Object of My Affection is in My Reflection: Coping with Narcissists by Rokelle Lerner. This book is newer but is extremely clear and insightful and has helped people since it came out 2 years ago.
I wish you the very best in this difficult situation!
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