Firstly, there is nothing in this man's history - as laid out in the original post - to suggest that he is in any way a danger to children. He had sex with his wife's (presumably adult) sisters and his adult step-daughter. These do represent some serious moral failings in him, at the time that he did these things. But can we not overcome our sins? St. Moses the Black was a brutal thief, adulterer, and murderer. Yet, he changed. He became a new man in Christ. I don't know how long ago this man committed his sexual immorality, but I do know that people can change.
I've committed horrific sins in my own past, things that I am so very, very far from proud of. If God judges me for the sins I've repented of, then I've no hope.