My cousins wife wants to have sex with me. There's no way I can tell him that.
Why not? That seems like something you shouldn't let pass.
I don't want to get involved in their problems.
That's understandable, but his wife wants to cheat on him. I imagine you'd want someone letting you know if your partner tried to do that.
I'm just gonna forget about it. Maybe they can resolve their problems on their own.
That isn't something you should hide and forget about. Your cousin deserves to know.