My 20-year-old brother is having intercourse with a 15-year-old girl

This is not the kind of thing I ever expected to be making, so if this is not the place for it my apologies. Let me know I will take it down. I know every girlfriend my 20-year-old brother has had and all of them broke ties with him after learning who he really is. put simply as a creep. But recently he started dating a 15-year-old. He's doing his best to hide it, but I have gotten multiple confirmations from both people I trust and Instagram accounts that they are dating. I recently learned from the woman who lives underneath him that he is being intimate with her and now I don't know what to do. I don't know what kind of information I need to obtain to do something about it. I've mentioned this to my father and I am going to talk to the mother of the girl today. Any information about how I could proceed without destroying my brother's reputation could be helpful. I'd also like to say if I can't get him to stop willingly I will call the police on him because in my eyes this is unacceptable. Thank you. I'm in the United States Indiana.