What? Concern for the victims? That doesn't seem very christian to me. Look at what leaders like the Elder Duggers did. Pray for the sinner. Put aside the ones upon which he sinned. Praise the lord.
I don't know anything about the person or the family, but this is quite a moral dilemma, which is why I phrased it as I did. I don't feel sorry for his parent, I feel sorry for their kids. I don't know anything about the family really, but it seems pretty strange from what I've read here.
And in this exact case when he is a minor, and the victim is his sister, I think it might actually be alright to keep it in the family, and not to tell the police.