Help! 9 year old with behavioural issues.

[Mummysarah123]

My 9 year old step-daughter throws tantrums just like a two year old. She screams, cries, throws things, chucks herself onto the floor, wets and soils herself, and makes herself sick. She lies about everything. In the past she has tried to strangle my six-year old daughter, ans recently tried to push her down the stairs. I cannot make my partner see that this is abnormal behaviour, and I am at the end of my tether. How do I deal with this?

Thanks for all the support and advice guys. I have (finally) managed to sit down with my partner and calmly discuss the situation, and he has agreed there is a problem and we need help. I wouldn't have been brave enough to do this without getting all your opinions and suggestions. As I was the only one who thought there was a problem, I was starting to think that I was the one who was wrong.
re; S's mother - she tried to put S into care because she couldn't cope with her behaviour.
I expected S to have problems after her brother was born, but they didn't manifest - she's been doing this since way before she knew there was a baby on the way, and had actually calmed down in recent months. I don't know what has sparked off this latest round of tantrums etc, but I am determined to get her the help she needs. I think the main reason I can see that her behaviour isn't normal is because my brother was very similar when we were growing up, and he has serious 'mental health' issues, as a result of not getting the help when he needed it.
I just want to thank you all again for showing me that I should trust my instincts in this matter x

It sounds like she is really lucky to have you in her life.