I am no defender of Josh and this whole thing makes me feel ill.
That said, if he had become some sort of advocate for victims and discussed his past abuse of his sisters, it would have dragged them into it. If my brother did something like this, I certainly wouldn't want him speaking out about what happened to me. Well maybe I could do it if I lived far away and my name was not used, but if we were this famous I would not give my consent. I might decide to come out with it myself at a time of my own choosing based on my healing, but to have him on the talk show trail when he felt like it would not fly with me.
He would need the consent of all 5 victims, IMO, before he went and did this. And this is where his parents screwed up (again). By making the family famous, they made it even more difficult for the girls to give consent. He couldn't talk about it without people knowing what happened to them due to their celebrity.
He should have repented, gotten himself help once he became an adult if his parents wouldn't when he was young, found some other way to do good, and live a quiet life, unless he had the express consent of all of his victims to splash their pain across tabloids.