dd1 is 12. She just got into a row with ds1, who's 11, about his belt. She was wearing his belt and he wanted it back. She wouldn't give it back so he started to get lary. I then said, 'you have to give him his belt back if he wants it' so she started taking it off extra slowly to wind him up.
He got more and more hysterical. She sort of kicked him but it was more pushing him away with her foot because he was getting very close and steamed up.
I said to dd 'OK you have to leave the room now. Go and get ready to go out'. (She's going to see Harry Potter with a friend.)She shouted, 'NO' and I carried on calmly telling her she had to leave the room. Eventually, when she still wouldn't leave, I said, 'I'm going to count to ten and you have to be out of the room' at which point she screamed, 'I hate all you little shits' and she threw her mobile phone hard on the floor. The screen smashed. (It's a touch screen.)
What went wrong and where do I go from here?