Younger Sister

I say you just watch her from the sidelines, you know like be aware of what she's doing, who she's hanging out with, but be careful never to interfere. Teenagers never react well to older siblings 'ruining their style' and tend to rebel against direct acts of interference so maybe a subtle "don't do anything I wouldn't do" approach would work. Don't tell her not to do this or not to do that, instead tell her stories of when you made a certain mistake and the consequences you had to face. Instead of putting the focus on her, you put it on yourself and that's a good way of not dictating her actions but letting her know of the possible outcomes to certain situations and letting her decide whether she still wants to commit the same mistakes or not.

It depends how close you are to your sister, and if she looks up to. You can give her advice and she'll probably take it to heart if she thinks highly of you. If not you can ask your mother, or someones she's close to explain what's good for her.

In my opinion, the one who is best to guide her in her adolescence period is your mother. Being a female herself and have passed that stage , she could very well relate to your sister. Besides, that's what a mother should do. Perhaps when it comes to boys' concern, then you can help her about it.