Help.

Someone did that anonymous note thing or wtv to harass my sister on Instagram. They said that they had been watching her and that she was sexy, etc. We dunno who this is, but we're guessing someone from our new school. We transferred 2 weeks ago. I'm genuinely tired of cleaning up after her shit. I don't know what to do atp. She's my older sister by 3 years. We can't just go around telling adults about this. My sister definitely won't allow it. I considered telling the principal about this but it hasn't been confirmed it was someone from our school. She found the account and questioned them, but they laughed it off. The stalker said they were a girl, but I don't believe it. They also said that if my sister sent them one pic of herself, they'd stop bothering her. I have no idea what they're gonna do if she sends a pic of herself. What if they make nud3s of her using ai? What the fuck do I do..