How to handle a 14yr old teen girl?

My sister is having a lot of trouble with her teen daughter. Her daughter has gone as far as pulling a knife on her mother she beats her younger brother she does it all. She doesn’t drink or do drugs as they r in a small area that is drug and alcohol free. She doesn’t know what to do anymore... does anyone have any... show more My sister is having a lot of trouble with her teen daughter. Her daughter has gone as far as pulling a knife on her mother she beats her younger brother she does it all. She doesn’t drink or do drugs as they r in a small area that is drug and alcohol free. She doesn’t know what to do anymore... does anyone have any ideas I can give to my sister that would help??? Thank u.
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