Make him promise that he should end the extramarital adventure at once and you would say nothing to your sister. Wait a prudential term of time and if he has not fullfilled, you must communicate it to your sister. That is the correct way.
Excuse me for my bad english, I am hispanic speaker. Leoleonard.
Would you want her to tell you of I were her. Be concerned about her not him.
If it's not your sisters husband then I wouldn't even get involved. Because if word gets out that your are being a busy body people might start acting different around you. You don't want that kind of feeling from people thinking if they do something wrong around you that you may sell them out.
But if it is your sisters husband then I would be all over it. But overall do what you feel is right.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.