Your sister sounds like me in my teens. I was a "skater kid" for a 3 or 4 years and I only listened to punk such as NOFX, Rancid, Pennywise etc. Eventually, through stuff like Zeppelin, Floyd and other classic bands I got into Anthrax(thrash) Metallica, Megadeth, Iron Maiden. These are the essentials to any person who is a metal fan. I do like alot of things other than metal but it is usually my prefernce.
Here's a basic list of cool metal song to check out:
Megadeth: Tornado of Souls, Holy Wars, Peace Sells
Iron Maiden: Hallowed be thy Name, Seventh Son of a Seventh Son
Anthrax: Caught in a mosh, antisocial
I too started playing guitar when I was about 13 and was in a band composed of all inactive witnesses. I left it a few months ago. We played at bars, local music festivals. No elders ever said anything to us about it. It wouldn't have mattered if they did. The only thing they ever did mention to our drummer was all the drinking we did. We have a local brewery and we have a kergerator in our practice spot which usually has 2 different kinds of beer in it. Nothing was ever done to any of us, just said they heard it was a problem. No problems here!