An angry, unhappy four year old. What to do?

I am thirteen years old, and I have a four year old niece who us along with my two sisters and mom.
We have all noticed her downright horrible behavior.
She screams when /anything/ doesn't go her way, cries all day, is constantly telling me what to do and what not to do, hits, throws herself around, and doesn't allow anything to go a way she doesn't want it to.
She is constantly yelling "I don't wanna!" or "But I WANNA" or if someone tells her to be quiet or something she will yell "YOU do that!", and she takes people's things figuring if she wants it the other person doesn't and she can take it.
   I noticed eairleier that she screeched/screamed at me, and started breathing incredibly hard.

  Her mother isn't ANY help. She is gone all day, is very young(twenty something) is angry anytime she is home and is constantly screaming at my niece. She swears in front of her too. If my sister is told to do anything, she will happily get in your face and yell at you to 'Never. Tell. Her. What. To. Do."
  
  My neice doesn't have much expeiance with other children, and somebody is usually watching her closy to make sure she doesn't hit anyone. She is unbelievably nice to kids though, just VERY 'in your face'. Litterly.

   What I am saying is, she has tantrums, is VERY stubborn, and wont listen to anyone.
   Nothing seems to work; what is wrong?

[margypops]

Firstly it is kind of you to come here and post this to get help for your niece she is a lucky girl to have you in her life and that you want to help her. I have a few ideas,.Does she go to school, who looks after her whilst her Mom is out working? She could do with some bounderies set, however I think that her Mom would be the one to do this. She also could do with more positive attention from her Mom, as it does read that there is a lack of Parent/child interaction. With some of the behavior to you ,maybe it would be a good thing to walk away and not yell back, dont feed into it, it is getting her negative attention.Focus on her positive side and praise her when she does something good. Is she involved in outside activities and Games,get her doing physical things with you all.If she cries a lot is she bored get her reading ,drawing and ask her Mom to give her more attention. Good Luck