I also agree with Lin. It is ultimately what will be best for you and your sister.
As far as the chances that he has done this to someone else or will do it as he ages, it is pretty likely he has and he will.
I did some research in college on sex offender treatment programs in the US and Canada. I have spent some time interviewing a woman who runs a sex offender treatment program in a Wisconsin prison. I have also interviewed a parolee. While some sex offenders (and it is a broad catagory) can be treated those who are pedofiles are most likely to reoffend, even after treatment. The man I talked to was a pedofile. He had been recently paroled. He told me he struggles every day. He didn't think like "normal" people. They use cognitive behavioural techniques to help these people. But still he "struggled". This man is back in prison after being out only a couple of years.
My heart goes out to you. What a heavy decision to make. My prayer/hope is that you will come to a decision that will give you and your sister peace.