Sisters

I wasn’t quite sure where to post this but I have a little sister who is 15 and I recently found an Instagram account she has that I or my parents didn’t know about. Some of the comments on her posts are disgusting, as in people saying sexual things or making comments to her. I’m torn on talking to my parents or just having a talk with my sister, what should I do? I don’t know if she would actually listen to me as she’s at that age she thinks she’s got it all figured out. I’m just concerned cause she doesn’t know these people and she’s 15. I need some advice before I do anything. I hate poking my nose in their business but it grossed me out.