Firstly, I do not speak as a victim of sexual abuse. For that kind of insight I recommend Joshua Roger's post.
Secondly, I do believe that God can forgive all sin by "the riches of grace" and the death and resurrection of His Son Jesus Christ (Ephesians 1: 7). And I expect we're all on the same page that Joshua Duggar did commit sin.
Thirdly, I do not agree with Christians saying that "he should be executed!!!" and baying for his blood. I do not believe in the death penalty (see my post on the Bali 9) and I think we, as Christ's followers, can do better than this. Ask God about it sometime:
"It is mine to avenge; I will repay." (Deuteronomy 32: 35a)
Some writers, such as this Geeks Under Grace post, go as far as to celebrate the testament of Josh Duggar's life as an exemple of redemption. I struggle with this, I really do. I think it comes across as insensitive to the victims and ignores the gravity of the situation. While I do not believe that was the author's intention it's hard to read it and lash out emotionally (note: I do not recommend this. Self-control is a fruit of the Holy Spirit [Galations 5: 23]).
Yes, Josh Duggar can and has (to the best of my very limited knowledge) found forgiveness and freedom in Christ. But that does not mean he shouldn't face the consequences of his actions through the justice system (no I'm still not going to say that we should execute him).
To face the court of law, albeit an earthly one, is the right thing to do in my opinion. The girls may not want to press charges (statute of limitations aside), and that is their choice, but they should have the option. It doesn't mean that their brother isn't forgiven by Christ or even that they themselves don't love him.
I am reminded of an episode of Press Gang, "Something Terrible," in which one of the characters discovers a young girl is being sexually abused by their father. This character shares his concerns with the main character and editor of the student paper. They decide to run a special edition on child abuse and are brainstorming some of the questions or concerns they should address:
Spike: I'm worried about my dad.
Spike: Maybe I still love him. I mean he is my dad. What's going to happen to him?
Colin: If your dad loved you then he knows he shouldn't be doing this, he knows he's harming you. And if he can't stop doing it on his own, then he needs someone to stop him.
I couldn't have said it better myself. I strongly agree with Joshua Rogers in saying that sexual abuse should not be dealt with internally (within the church): go to the police (which the Duggars did), do not cover it up, do not sweep it under the rug. They can find forgiveness and redememption in Christ while still allowing justice to be served.