Thank you for the additional information. It helps.
I agree with you, all the signs you listed sound like abuse. Anyone who yells and belittles another person is being emotionally abusive at the very least. And if he is saying things about you as well, that shows he feels threatened by you. That may mean that he fears your influence over your brother and the possibility that you may make him leave. Anytime an abuser is challenged, they will most likely react with hostility because they feel threatened.
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 brother's partner may be using psychological abuse to convince your brother that he 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. 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 brother 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.
One of the best things you can do is be supportive of your brother without trying to convince him to leave. If he feels he can come to you without having to defend his relationship, then he may grow closer to you and may even share his feelings with you. When that happens, you may be able to gently introduce the idea that he is being abused. Even providing him with safety numbers and resources about domestic violence might help.
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 brother. 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 brother with resources in your area.
I hope this has helped you,