I totally get "wanting them to understand" - but that needs to be the conversation you have with your son. Trust me, I know first hand what this feels like - any of us who are married to a man that acted out sexually knows how badly we want to qualify the behavior - but it is just NOT our job. That's why we support each other. Because here, you can qualify it
Do you know how hard it is to look in the faces of my friends who know what my husband did and not be able to tell them the whys?? I think you do know
The thing is - it doesn't matter if they understand. In fact, they may not really understand. It may backfire for you. You just can never tell - and this is why your son needs to decide who he would like to tell, if anyone.
As far as warning your niece - tell her a hypothetical story. Tell her that this type of abuse is most frequently at the hands of family or trusted people. Tell her the statistics. Educate her.
Focus on your son and his healing. And your healing. This is very painful stuff and I am sorry you are going through it.
Just my two cents... from experience.