What should i do ? my sister has shagged and is going out with a guy i hate.?

i cant stand the guy my sister fancies he has been running his mouth all over town and my sister drank herself into hospital because of him . his mates have been haring me in the streets about it . im a heck of a lot bigger than them although they are my age , im 6ft2 and 15 stone and definatley not fat , im so flipping close to knocking one flat