Well first since your daughter is the victim in this situation I think that it would be best to follow her lead. It seems as if this is something that she wants to move on from. So just allow that to happen. Tell her that you are here for her in case she wants to talk about it, and even ask her what you can do in order to make things better. But if she is ready to move on then accept that, it may be that she has dealt with the situation in her own way and she may not want to re open wounds or revisit the past.
As for your son, it was not acceptable for his to molest his sister at all. But it is possible that he may have just made a mistake. The things is that as adults this is something that your son and daughter need to deal with. If they both have settled the situation the last thing you want to do is to get involved. I know that as parents it is an urge to get involved, but you have to fight this urge because you getting in the middle may make it worse for everyone. Possibly even his future marriage. Like I said, he may have just made a mistake when he was young. If as an adult he is continuing to molest others, then this would of course need to be brought to the attention of others.
But lIke I said the best way to handle it is to ask your daughter if she would all like to talk as a family or how she best feels that she can be helped. And as the victim, follow her lead on what she wants to do. If she wants to leave things as they are, then just do that. If she wants to talk as a family then be prepared to do that as well. But only re-open the past, if it is okay with your daughter, since she is the one that is affected the most.
I know you want to get more involved, but sometimes if the wounds are healed, it best to let them be healed and not re-open them by bringing them up again. It seems like your daughter has dealt with the situation and is ready to move on. It also seems like your son feels remorseful and does not seem to be a habitual sex offender. So for now, just be there when she needs you and try not to force the issue too much.