Thank you for the additional information.
In your niece's situation, she is going to have to want to change before any treatment is going to help her. Although it would be nice to be able to control her, it is not possible. What your niece's husband can do is control how he reacts to her behavior and hopefully through what he does it will affect her.
When someone develops coping mechanisms at an early age like your niece did, they become use to relating to the world on that level. Although your niece may have learned how to respond to the many problems of her childhood with this behavior, it no longer works now that she is an adult. What it does instead is cause problems. But she may not see that since it is the only way she knows to act.
What your niece's husband can do is try to break through your niece's denial that she has problems. He may want to try a family intervention to start. This is when all family (and friends if they want to be included) confront the person in a group and tell them the effects their behavior is having on everyone. This is done with a therapist or intervention specialist so the intervention is fair and runs the way it is supposed to. By doing an intervention, your niece will be exposed to the people she has hurt and she can see the consequences of her behavior. This may help break through her denial and start her on the path of recovery.
Your niece's husband can also try therapy for himself (and children if there are any). This will help him learn ways he can cope better with her behavior and also it will provide him with support until he can find a solution. To find a therapist, he can ask his doctor for referral. Or he can search on line at http://therapists.psychologytoday.com/rms/.
If your niece's behavior as damaged the marriage, he may want to consider a separation. This may help his wife see that her actions have had consequences. It also may serve as a wake up call to her that she cannot go on behaving as she has.
I hope this has helped. Let me know if you have more questions,