She is not responding to the time out step.
She is doing the following:
throwing toys at DS. Not in anger, just seems like because she can, quite casual about it.
Throws her toys down the stairs/throws them generally.
hits DS with something, not hard enough to hurt, but enough to make him and me go 
Ignores us, almost like she does not hear us. Not necessarily rudely, just chooses to continue doing something we ask her not to, or not do something we ask her to.
Shouts at her brother.
It has all started quite recently, in the last 2-3 weeks? just a minute ago I asked her if she could pass a toy to DS, and she started playing with it and pushed him away. i asked her again, politely to pass it to him and she threw it at him across the room (lukcily it missed him). I told her go and sit on the 'step' and she refused, casually walked across the front room, and stepped on each of her three small wooden dolls (from the dolls house), carefully, and on purpose.
Yesterday, she swung on a shelf in the front room which she knows she can't do, and it fell off, breaking several ornaments and almost landed on DS.
She is on the step and I have taken the dolls off her. She is now playing with a toy she has picked up, and is chatting away, seems not at all bothered, kicking at the wall.
It has been a tough few weeks in terms of tiredness in the pavlov house, but she gets plenty of (positive) attention. I expect it is all in response to how things are atm but I do not know how to get her to start behaving.
We have been using positive rewards for good behaviour, she gets a gold star for very good behaviour, such as eating all her dinner with knife and fork and not using her hands (a normal thing for her!) and not making a mess, for being kind to her brother, for being kind and lovely when we were poorly this week, that kind of thing. When she has 5 stars, she can get a small gift of her choice, out of two or three things (she wanted just a lollipop last time! but a littliest pet shop animal or something like that).
She responds well to this, but her negative behaviour is becoming increasingly out of control. Not sure how to address it, any advice gratefully received.