A sister's friend

Sometimes I feel as if I could kill someone. What had this world came out to really become? Selfish, angry, a disappointment? Maybe it isn't the world, it just the few people who live in it that show their true colors once they've got you in your grasp. My sister has a friend that you would think is a pretty nice kid. She's quiet, she smiles a lot and she just nice.... well, on the inside. Now I don't see my sister or her friend much because I have basketball practice right after school, and I don't get home til about 6:30 p.m., but I have heard of all the things that she does to show her true colors. She rude beyond rude. She says mean things about our family and gossips like 3 old ladies. Now, you must understand that my sister is a very quiet and shy person as well, she's not all up to saying things to someone if she disagrees unless it's me. :) But the words that she can say and the unkind sound in her voice, it's just not right. What exactly am I suppose to do though. I never see them or else I would catch her in the act and have a big chat with that girl. Maybe I could tell my sister to somehow use a video tap or recorder whenever she's around her friend that way I can have proof when her friend goes crying to her mom saying that I punched her in the face. Nobody talks about my family or to my sister like she does. That girl should be put in rehab or something.