It really sounds like permanently disconnecting from these people is the only way to go in order to live a life of your own, and to regain a sense of security. They do not sound like people who would respond to reason, or have a sense of responsibility or fairness.

I am always amazed by, and afraid of, people who spend their energy trying to hurt others.
The nieces and nephews will grow up, and be able to make their own decisions. If you do see/suspect abuse you can make a confidential report to children's services.