Listen, I know child molesters are a really iffy topic in this community but when I posted I was keeping in mind a few things mainly that I'm reading from a COMPLETE outsiders perspective. I'm reading a post on a forum! I would be hesitant to give someone such concrete advice that could alter lives when I don't really know 100% of the story, and the truth. I don't think OP should go out and high five the guy, but I also wouldn't go ahead and call the police. I think she should just talk to the guy, I mean what not. She doesn't have to have her kids around him, can go ahead and tell the step-Dad, and can tell her sister he's not welcome but I would ALSO talk to him. Ask him why he hide it, etc. I mean why not?
The truth (I think) is this: anyone who's had a bad experience with a molester of any kind is going to be very much inclined to call the police on him. I'm just saying TALK first before making any decisions