Hi, I'd like to help you with your question.
Your sister may be having trouble with her seeing her son as he is because she does not want to believe her son could do harm to others. Many parents want to believe the best of their children. And even when their children do things that are bad, they still refuse to accept what they see. To admit her son has done these things, your sister would have to face how he got this way. Most parents will first blame themselves for their children's shortcomings, and these feelings may be to overwhelming for your sister to deal with.
Also, if your sister's husband was a sociopath as well, your sister may be a co dependent. A co dependent is involved in a relationship where the other person is abusive. The co dependent becomes a victim of the abuser, who uses emotional and sometimes physical abuse to demean, slander and harm the victim. It can become so bad that the victim is unable to get themselves out of the situation.
Your sister may also fear that her son would become violent and hurt her or someone she loves.
You can talk with your sister about trying therapy. A therapist may be able to help your sister see the damage her son's behavior has had on the family. Your sister may be willing to go with you and any other family members that want support in dealing with what your nephew has done to the family. Family therapy and individual therapy can provide support and ways to cope so the family can move on.
There are also support groups and self help resources that your sister and other family members may be willing to try. Here are a few to get you started:
Why Does He Do That?: Inside the Minds of Angry and Controlling Men by Lundy Bancroft
Finding Your Way Through Domestic Abuse: A Guide to Physical, Emotional, And Spiritual Healing by Connie Fourre
In Sheep's Clothing: Understanding and Dealing with Manipulative People by George K. Simon
You can find these books on Amazon.com or your local library may have them for you.
The more you can learn about the effects of abuse on a family, the better you will be able to talk with your sister and possibly help her find a way to deal with her son.
I hope this has helped you,