My instinct on this tells me that your step daughter is seeking attention......if she is still grieving for the loss of her mother, then this is inevitable....the grieving process can take up to two years, and can take may forms. She is crying out for attention from someone somewhere.....and the only way she appears to get it is by being naughty. Usually what happens is that when a child is being good they are rarely rewarded or get attention for good behaviour...but when being naughty they get all the attention they need, although the attention is negative, it is still attention to the child nonetheless.
My advice to you is to start rewarding the good behaviour, and if she does a minor violation, try to ignore it. In a way, its like a two year old that has tantrums. the only way to kerb these is to ignore them, and they soon dissapear. In other words, dont feed the negativity of the child....just give rewards for good behaviour. if your step daughter is in breach of a serious violation then you cant ignore this.....but this behaviour has to be dealt with in a certain way also......no hitting, no name calling etc, but calmly tell the child what she has done wrong and find an appropriate punishment, such as going without pocket money, or confiscating a special toy, but on no account condemn the child to its room as a form of punishment, as this should be the childs place of sanctuary and privacy.