Why did my sis get mad when I kissed her boyfriend? 10 points TY!!?

Ok so I'm like way hotter then my sister and when her boyfriend came over last weekend I kissed him because I thought he deserved way better than her. He kissed me back and stuff. So her boyfriend stayed the night and we had and my sister was watching us the whole time. My sister got so angry, she blamed it all on me and called my boyfriend and told him what I had done, then yelled at me for trying to steal her boyfriend. I told her that her ugly azz was the reason I did that- and that he deserved more than her. She started sobbing and kicked her boyfriend out, then told mom and dad. They grounded me! What is my families prob? TY !!