I would honestly take my lead from how her boyfriend feels. Does he feel the same way you do? Regret, showing remorse, feeling like the disgusting pig he is?
If you BOTH are feeling horrible about it, I'd say not to tell her. This was a mistake, but you are showing remorse and regret. However, if he's showing no regret and acting like he's finally conquered some sick life-long goal, I'd tell her, because he's going to cheat on her again.
Either way, there's no doubt that you messed up royally and now have to live with the guilt and the constant pressure of never telling her.
You don't take your lead from how the boyfriend feels. What a silly answer that was. The best answer is NO.
If you love your sister, you will NEVER tell her. She'll hate YOU as well as the boyfriend. You and the boyfriend should never do it again but you should also keep your mouths shut.
Whether or not you or he feel remorse is also irrelevant. Keep your mouths shut. There's no sense hurting your sister.
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.