In some states if a social worker and a psychiatrist decide someone should be hospitalized for 30 days, it is possible.

It sounds like your niece daughter is in danger of being abused mentally and emotionally. Is there anything you could do to provide a safe haven for her or help her find a safe place to be?

The picture you describe of your sister sounds like someone who needs professional help.

In many states the state police will bring someone to a psych hospital if they are a danger to themselves or others.