Son was just curious

My son recently told me that he had "played around" with my daughter when they were younger. At the time he was only about 10 and she 8.
He told me he just did it because he was curious, and he called them "experiments". He says he recalls (he is 20 now) that the daughter was laughing most of the time, and did not refuse him. He never told her that she should not tell. There was no real touching involved, just looking.
It is years later, but my daughter is showing signs of typical molestation victims: listlessness, authority problems, solitude. She is never violent, and has not submitted to any drugs, and I feel that it may not be caused by this previous act. It only continued for a short while, much before my son was even a teenager, possibly less than a year.
My son is a wonderful person and has never done anything of the sort again. He has looked at p 0 r n o g r a p h y rarely but I don't see this as a problem as it is common among teenage boys. He has had girlfriends and none of them were sexually active or agressive in any way that a sexual abuser would act.
I don't feel my son is an abuser, and I don't think he is a bad person at all. He has yet to talk to his sister about this for fear of destroying the little relationship they already have. My daughter is somewhat removed from everyone, her father and myself included, as well as her brother, so she does not seem to show extensive signs of disgust for him, and especially not fear of him.
I don't really want to bring this to a psychologist because I have heard that it needs to be reported, and I know that people will get the wrong impression of my son if this gets out. Please someone tell me that this may be a slightly abnormal thing. I am not worried about my son at all, he has never done anything wrong like this past that point and won't again, and he was never molested as a child or anything of the sort. I am especially worried that his life will be greatly turned for this worse if this gets out about him, because people will take it the wrong way. It does haunt my son to think about it, and that he may have caused this emotional distress in my daughter.
It is also hard to talk to my daughter about anything, much less even think to bring this up.
Please give feedback, if you've had this same situation, or any advice. Thanks for your time. And God bless all of you who have much more violent and painful dealings than has happened to my family. Tweet