Hi, I'd like to help you with your question.
I agree with you, all the signs you listed sound like abuse. Anyone who is cruel and manipulative to another person is being emotionally abusive at the very least. And if your sister in law's husband is forcing her to cut off contact with friends and family, that is a sure sign that there is abuse going on.
People who are victims of domestic abuse often have a very hard time seeing that they are indeed being abused. Part of this is because they have a low self esteem. They either entered the relationship with low self esteem or through repeated abuse developed a low opinion of themselves. This makes it hard for the person to leave because they feel they deserve the abuse.
Also, your sister in law's husband may be using psychological abuse to convince her that she needs to stay. Controlling the victim is important for the abuser. If the victim would realize that they could leave, then the abuser would lose the relationship and the control they have over the victim. In order to make the victims stay the abuser will not only try to lower their self esteem but they will use fear and threats to get them to stay. So your sister in law may feel powerless to break free of the abuse.
Many victims also defend their abusers. This is as a result of the psychological abuse and the low self esteem they develop in the relationship. They feel they cannot live without the other person and are convinced they would not do well on their own. Also, many domestic abuse victims have dependent personality traits, making it even harder for them to be convinced they can live on their own outside of the relationship.
Therefore, it is very hard for someone from outside of the relationship to convince the victim to leave. Many will fight harder to stay in the relationship and may even bond more with the abuser, creating a "us against them" mentality. Your sister in law may also be suffering from something called Stockholm Syndrome, where the victim starts to sympathize with the abuser.
If you can still talk with your sister in law, one of the best things you can do is be supportive of your sister in law without trying to convince her to leave. If she feels she can come to you without having to defend her relationship, then she may even share her feelings with you. When that happens, you may be able to gently introduce the idea that she is being abused. Even providing her with safety numbers and resources about domestic violence might help. Whatever happens, try to maintain contact with her. She may someday need to turn to someone and you might be the only person she has left.
You may also want to express your concerns to other family members, even if they do not live nearby. You need support and they may be able to help.
Also, learn what you can about domestic abuse. The more you know, the more you can help your sister in law. Here are some resources to help you:
You may also want to contact the local domestic abuse hotline for more ideas on how to help your sister in law with resources in your area. And also contact the local domestic shelters for ideas of how to approach the situation.
I hope this has helped you,