Obviously his sisters forgave him long ago. Why can’t you?
I can guarantee you that those girls are doing the "godly" or "family" or "Christ" thing and "forgiving" him. It's what's expected of them and they are doing it. And they're probably trying to forgive and put it behind them.
I don't for a minute believe this is over and done with in their minds or souls. As they get older and more mature and it fully sinks in what was done to them in their own home by their own brother and the parents didn't go full gonzo on their pedophile son, THAT is when it's going to hit this family hard.