K.B.
Hi J.-
When I was younger I listened to many bands that my parents probably would have been shocked to hear. To be honest I probably would have been embarassed to listen to the bands with them. But I was a straight A student, never got into any trouble and was very trustworthy. We never know what would have been if things had been different...perhaps if my parents had checked up on my music regularly I would have simply followed their rules, not listened to the music, and I would have still been a "good kid" and everything would have turned out the same. OR I would have felt that they didn't trust me, I had to sneak around behind their back, and I might have rebelled more. It's a tough balancing act and I'm not quite sure how I'm going to do it when my children are approaching the teenage years.
I guess I feel that as long as your sister isn't changing for the worse since listening to this music and it just seems to be a phase use it as a opportunity to discuss what she is hearing. There are some things mentioned in their songs (domestic violence, drugs, etc) that actually could provide a starting point for a conversation about this as she gets closer to dating and being exposed to these things.
Of course you should give her your opinion on the music without being judgemental. Tell her you don't like the music because of ......... It could be that she will respect your opinion enough to stop listening on her own.
I think that as her sister it's not your place to "step in" on this. Again if you see her doing things she shouldn't be or notice a change in her personality I think it's fine to say something to her or you mom as a concerned sister, but to police her music choices is not one of your jobs. You have a unique opportunity to be an adult her life that she can confide in and look to as a friend, not a disciplinarian or judge.
Good luck,
K.