need help child seems to be getting worse

I have a 10yr old step daughter who is the sweetest most beautiful lieing, stealing, intimidating, manipulative, threatning and sexually exploritive child anyone would ever want or not want to meet. She has been in our house for about 18months. In that time she has been caught touching herself in front of my 4yr old son, then it came out that she taught him how to "hump" according to her he taught her. She has been caught threating him over and over again, she wispers to him in a low mean voice and then you hear his saying no no please dont, and when questioned she looks up at you and says what I didnt say anything to him I swear to god it was not me. She has stolen a phone from her dad and lied about where it came from even after she was busted, she stole a boy at schools homework so she could get a doughnut, she even went as far as to steal her grandmothers cell phone with everyone in the house and said she didnot do it one of us must have taken the phone and put it in her pocket to get her into trouble. She can turn on the waterworks at the drop of a hat and swears to god she is not lieing even after she has been proven wrong that it is a lie, she commits totally to her lies and holds on to them for hours even after you show her the proof that you know she is lieing. In the last couple months my 4yr old has started wetting the bed, something he has not done in 2 yrs, he is biting his finger nails down so far they have started to bleed and he is obsessed with his penis now I am not sure if these things are normal for a 4yr old boy but I am afraid she may be doing things to him that I do not want to think about. in the last month he has started freaking out anytime his dad and I leave his sight as long as 1 of us is there it is ok but if we leave both leave he gets very upset. He has never done this before he has been in daycare since he was 4 months old. He even wants to sleep with us now,  WHAT DO I DO? Thanks for any help

[Kevin Kennedy, Ph.D.]

Some emotional disorders, such as anxiety, depression and bipolar disorder, have a strong biological base and heredity generally plays an important role in such conditions.

[Kevin Kennedy, Ph.D.]

What you do is get some serious help right away for this little girl. She is showing signs of serious emotional disturbance and this is a crisis. Your son is being victimized and showing signs of trauma. There is no time to waste - seek help asap.

[closetodone]

We have found a therepist who specalizes in abuse and a whole list of other things that I believe will help our famil. And we keep the children separated as much as possible, they are only allowed to play together in the livingroom were we can monitor every second. Her mother is said to be a pathelogical lier and bipolar is it possible these things are inheritaded from mom?

Thank you I was worried I was starting to make to much of all of this that and she couldnt be hurting my boy.