Hi, I'd like to help you with your question.
In a situation like this is, it is hard to stand by when you see this type of abuse going on. The fact that your nice does not see it and doesn't help herself is most likely due to the abuse she suffered as a child. It is very common for adults abused as children to believe that they deserve to be treated badly. It is also hard for them to see what is wrong with the situation since they have not been exposed to a loving and caring environment.
Your niece may not be willing to get help. To face what she has been through and is still going through in her marriage is difficult. It would make her face her pain and she would then have to do something about her situation. Confronting her about it may help, but only if done in a loving way. If your niece is willing to get help, suggest she see a counselor. Her doctor can provide a referral or if she attends church, she can talk with her pastor. Also, she can search on line at http://therapists.psychologytoday.com/rms/.
Also, you can educate yourself (and her, if she is willing) about abuse and how it affects survivors. Here are some resources to help you get started:
http://www.ascasupport.org/- they offer support groups and other resources for those who survived abuse.
a great resource guide for those who are suffering with domestic abuse.
Adult Children of Abusive Parents: A Healing Program for Those Who Have Been Physically, Sexually, or Emotionally Abused by Steven Farmer- a great book to help her understand why she feels the way she does.
An Adult Child's Guide to What's 'Normal' by John C. Friel Ph.D. and Linda D. Friel M.A
You can find these books on Amazon.com or your local library may have them for you.
It is great that your niece has you for support. One of the most important things you can do for her, especially if she is unwilling to get help, is provide a safe place for her to turn to if she does choose to get out of her marriage and away from her abusers. Tell her to call you anytime and each time you have contact with her, reinforce this message. This will help give her strength if she finds a way to face her situation and get out of it.
I hope this has helped you,