It is natural for people to react with anger when taunted. When you feel this way, it is important to de-escalate the situation. Walk away. If you cannot physically walk away, do it mentally. Always act calm even if you do not feel it. When she is screaming it is the same as someone else vomiting. Please take a NAMI family-to-family class. It is free. It will help you. Many people have an emergency medication to take when they get that way. Does your niece have an emergency calming medication you can give her when she gets that way? Also, maybe you should read the book called "It's Not Mental" by Jeanie Wolfson since that is also about a child who had a severe mental illness. But maybe it is possible that your niece has more than bipolar. Maybe she also has a real personality problem and really is becoming a bad person. Bipolar does not make a person steal. It does not make a person taunt another person. Maybe it will predispose someone to do that when they are manic. Was she manic? If so, then the solution is better control of her symptoms. But maybe this behavior is not due to symptoms. It is very hard to know the difference.