Why the hell does my 12 year old sister scream so much?

All she ever does is scream at me, scream when things go wrong scream for no real reason. For example she will think the cat is going to pounce at her but will be crouched down starting at the cat and start screaming and be like "get away from Me, NO DON'T" scream scream scream. Or if we are in public (for example I will pick her up from school and won't go with her to some shop) and I do something she doesn't agree with she will yell and swear at me and start crying and I will just be standing there like huh? Or at home she will be told to go to bed at 9:30 and she will decide that isn't right and she will have a screaming fit at who ever told her to and the way she screams and the things she will yell is like you are beating her. I swear one day someone will call the police on us, also the only way to get her to shut up is to scream louder then her and tell her to shut it. Then she is all like "why do you always scream at me, you hate me, you are SO MEAN". Also no matter who she is mad at she will always take it out on me like the other day she was mad at my mum so my sister tried to choke me -.- or I will be lying down and she will jump on me, she always hit and punches me I know she is only 12 but we are pretty much the same size. My parents think she might have mild aspergers so they go easy on her personally I think they raised her to be a brat and there is nothing wrong with her. The way she behave towards me has nothing to do with possible aspergers she knows exactly what she is doing.
Why the hell does my 12 year old sister scream so much?
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