Dh and I (and other dc's) are really struggling with dd2's behaviour at the moment.We have 3dc, dd1 (14) dd2 (7) and DS (5).
There are no problems at school (have spoken to her teacher), she is kind, helpful and a joy to teach
. A little too chatty but this is improving.
At home she is like a different child
. She is rude, defiant and thoroughly unpleasant to be around at the moment. If we ask her to do anything she is rude and she refuses to take no for an answer, ever. If I say no to her she will argue, and even if I reiterate that the answer is still no and I am not going to negotiate or change my mind, she escalates to shouting and screaming and throwing things.
She constantly tells hers younger DB not to do things, or encourages him to do things to get him into trouble, then laughs when he is told off.
We cannot leave dd2 and DS alone in a room together as invariably there will be a disagreement which quickly escalates into a fight.
Dd2 has always been an acquisitive child, who doesn't seem to be able to share her toys, so for example if DD and DS are playing with Lego, she will take more of the bits/ all of the bits DS wants and scream at DS if he touches them as they are 'hers'. DS is usually quite amenable but this is starting to annoy him as he can never play or do anything without dd2 taking over.
There seem to be no consequences that she cares about, she can lose her pudding after tea, her bedtime story, screen time or go to bed earlier than her DB, nothing seems to motivate her to behave well.
We have tried time out, removal of treats and privileges, ignoring bad behaviour and praising good behaviour but we are running out of ideas.
She currently shares a bedroom with ds, but wakes up early and then proceeds to wake him and the rest of the household up by starting fights with ds and screaming at ds.
Has anyone got any ideas? We are getting to the point where everyone in the house is bad tempered constantly, and however hard I try to remember it's the beviour not the child I am really struggling to enjoy spending any time with dd2 as it's always a battle 
It feels like you could offer dd2 the world and it wouldn't be enough. The only time she is happy and pleasant to be around is if she is doing something she has chosen to do and all the attention is solely on her, we do spend time one to one with her as much as possible, but it can't be all the time as we have other children ( who incidentally are nothing like this!).
I grew up with a shouty, emotionally (& physically) abusive mother and I don't want to be that strung out, shouty woman on the edge that I feel I am turning into 