Hello. I agree with you that treating her the way she treats her brother would probably not work and it is also a bit cruel. I have children of my own and have experienced "sweetest kid in school" followed by oppositional behavior at home. I see that you have already tried giving her consequences and letting her think about her actions as a sort of time out. Have you tried praising her for the positive behavior? Sometimes children will react to praise and positive reinforcement more than to the negative consequences. I would suggest saying something, like "That was so nice or sweet of you" when you "catch" her and her brother getting along. You could also try engaging them in some type of family or child activity where they would have to get along and then praise them for that. For example, having them work on a special project together or a game that is non-competitive. Of course you would be supervising this and providing positive feedback when needed. I'm glad that you are taking your own "time outs" and not lashing out, as this might make her even angrier. I do understand this can be very frustrating, as my own children are 8 years apart and still bicker constantly, but I have tried the positive reinforcement and rewards strategy both at home and with other families and it has worked well. Please let me know if you have any further questions that I can assist with. Or if you are satisfied with my answer please click accept. I am here to help, so please feel free to contact me.