I hate my cousin sooo much?

I hate my cousin so much!!! When I go to my grandmother's house she always says rumors about me like she goes to my aunts and says "You know, Merrisa always talks about you and tells my other cousin what you said about her" And then my cousin that I hate comes to me and says that she told my aunts "for my own good" and she told me to stop going with my other cousin that I like and that is much much nicer. Everything that I or my cousin say she's gonna change everything about it and tell my whole family AND IM SICK OF IT. And every time I open facebook she's always peeking when I type my password and if im talking to my friends or my boyfriend SHE WONT LEAVE US ALONE! She reads everything that is NON of her business between me and my boyfriend. Also when she was using the internet she was like "can u go away for a bit? and mind ur own business" AND LIKE SHE DOES!! So I did because I dont want to be nosy like her, but then I got curious and I peeked a bit from her back and I couldnt believe what she was watching!!! She was watching PORN!! And I started shouting at her because she was just 11!! (Im 15 btw). So thats her and I-HATE-HER so so so so much but she wont stop following me and copying what I do, and another thing, she wont stop lieng. And if I fight with her, she always make rumors about me and tell my WHOLE family, and im staying here for 3 weeks :'''( PLEASE HEEEEELP