Why is my sister like this?

I am 18 and my sister is 16. She is very outgoing loud and dramatic and I am very shy quiet and down to earth. My sister fights a lot with my parents over the stupidist things. She has on occasion gotten pissed off at me and dumped coffee on me and thrown things at me. She even hit my bf with a remote because she thinks he is annoying. She gets into trouble a lot because she fights with my parents and she gets mad because she thinks they don't punish me when I do something wrong when actually I don't get into a whole lot of trouble or fight with my parents really. My sister hates my boyfriend and complains that he is controlling, selfish, and demanding which is not true at all. She complains every time he comes over but we actually just straight up avoid her cuz we don't want to deal with her. And even if she doesn't like him she knows that he is the only person I am really close with so why can't she just leave it alone and let me be happy? She also claims that I am self centered and I always make things about me. I mean seriously I am an extremely quiet person and when I am with a group of people they forget I am even there. My mom says its because my sister is jealous of me but why does she have to be so mean about it. I snapped a few weeks ago after she called me a f***ing self centered b**** and got into a physical fight with her which was wrong of me and I am way to old to be doing that. She was pretty pleased though because it was the first time I have ever gotten grounded. It has gotten to the point where we can't be around eachother.