This is a legit question

I graduated with a girl last year, and her younger sister is in 7th grade with my younger sister, & they were talking in class today & her sister told my sister she had her sister pose NAKED to send pictures to this boy she liked. The girl, is 19 years old, and her sister and the boy are hardly 13. I'm disgusted, but going to the police won't solve anything, because the police in my town are worthless.. my question is, what do I do?? Do I tell the county police?? Or the kids parents??? I'm not going to do nothing. That is sick and disgusting.