My sister is racist... what do I do?

I'm 18, my sister is 16, and she's racist towards Asian people. She will never use the word 'Asian', she always says 'paki'. She's a real shitty little brat. Not long ago, a new girl from Kuwait started late on my college course, and I invited her back to our house after college to hang out with me and another friend. As soon as my sister saw her, she went into the kitchen and said to my mum "Has she brought home a paki?!" Most of my friends are Asian and since then I haven't brought any of my friends back, because I know she'll be making racist comments about them behind their back. Just an hour ago, my mum was on the phone to the orthodonist my sister goes to, (Her teeth had move funny apparently, and wanted an explaination) and she shouted for the receptionist to hear "Tell the paki to go back to his own country!" (Her orthodonsist is British Asian). She makes me sick, I'm ashamed to be her sister. What should I do?