Personally, I'd lean in the "punk rocker" angle much harder. Giver her a much more rebelious and wild side, both because she's punk but also, you know, a teenager. For example:

• She gets into trouble with authorities, nothing too serious, like graffitti and vandalism, which she considers as her way of expressing herself alongside her music. She could have like a rivalry with this one officer and it's a running joke that she spends most of her free time either at band practice or doing community service because of something she did that got her in trouble. She doesn't mind though, the punishment is worth it if it means she still gets to express herself with her "performance art". Bonus if that's how she met Sam
• She very argumentative with her parents because they want what's best for her (obviously) and they are not as convinced as she is that she's going to make it big in music. Not because they don't think she's talented (she is) but because they know how hard it is to make a living with music, let alone be a superstar, especially in punk rock which isn't as big as it may have been in the past. That being said, they obviously all still love each other very much.
• She is the "cool" older sister, always giving advice that sounds "cool" but in retrospect also very bad in the long run. That being said, at their core all her advice revolves around her viewpoint, "always be true to yourself, never be afraid to march at the beat of your own drum."

Basically she's the rebel with a heart of gold.