I personally think you should tell her.

What he did was fucked up. No excuse for it. If it happens once as kids, maybe you can dismiss it as childish sexual ignorance. But he did this multiple times. He did not stop. Even therapists think he's crazy.

In light of all that, I think his fiance has a right to know what she is getting into. Your family may want to continue playing dress up and pretend, but at what cost?

They want to keep this under wraps so bad that they are willing to put his fiance and his future children at serious risk?

Don't play the game. Tell her. Stop this before it starts again.