What should I do (I'm guilty too)?

I'll keep it short:

- My brother dated his his girlfriend for 4 years

- I found out she's cheating on him with 2 guys

- She said if I tell him, she'll tell (she has evidence of it) what happened 2 years ago

- 2 years ago, we had a ONS and promised to never speak of it, never did it again

I really want to get it over with. She's a cheating skank and now I hate her. Yet, I did him wrong too. He's going to break up with her for sure but then our relationship will suffer too. So what should I do?

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I'm screwed. If I tell on her, she's going to tell on me too and show the evidence.
What should I do (I'm guilty too)?
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