[Potentially NSFW] What can you do if you come across someone online bragging about committing a crime?

Saw a post online claiming to be a 20 year old guy who had “seduced” his 14 year old sister. Obviously anyone can say they’re anyone and have done anything on the internet and there’s no way to know if this is even true. But I want to know, what steps could a civilian take when this kind of thing happens? I just feel sickened thinking there might be a fourteen year old girl out there being abused and I sat back and didn’t do anything.