How would you tell a friend that you slept with her younger sister's boyfriend?

Here’s the deal: I know it was wrong, so save the judgement. It was a weird Ross and Rachel “on a break” type thing.

They weren’t together. We hooked up. She’s 20, he and I are both older. When it happened about a week ago, we decided not to tell anyone, since they had just “broken up”. We figured it was best if no one found out.

Since then, they’ve gotten back together. Now I feel some moral obligation to at least tell my friend, so she can talk to her sister. He does not want to tell.

At this point, would it benefit anyone to find out? Undoubtedly, it will make things very complicated for a while, I’m not an idiot, I realize that’s one of the consequences of my actions. I just don’t know if it would be worth the consequences.