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I know how that feels, like many many others, I've been there too. The key thing to gain some balance is to realize that it HURTS NOW, because it's as if it just happened last week. Have you talked with her about how she felt after it happened? It could ease your present pain if she'd tell you about some of the struggles she has had with her memory of it over all those years. For she has probably suffered more from it than either of you would have if the truth had come out by either her or your husband confessing it 20-30-40 years ago.
She was probably trying to protect the sanctity of your marriage in YOUR mind, because she cared about you. And if some conscious or unconscious sibling rivalry played a role in her failure, that can be digested too. The important thing now is that you two still have each other, and you won't want your present love and trust to be destroyed by what happened so long ago.
I think that if you can get her to talk about it--NOT the details of what happened (because you DON'T need ANY corrosive images to stick in your mind's eye), but how she and your husband had to deal with it in themselves, and how it makes you feel now, since it feels like it's just happened because you just learned about it. Let her tell you about her struggles with that secret, and let her feel empathy for what you're going thru now
Let her feel the remorse she's probably been trying to keep away for so long. Don't rush to forgiveness until you've aired all your feelings (and that may include several "aftershocks" days later). But the result can be a better future, at least better than right now. Just imagine: If she could have kept the secret until SHE died, you would have been spared what you're feeling now. Your husband kept it secret, so he didn't want to hurt you and have to see your marriage pay a price for that. But it's obviously burned a hole in your sister's heart, because of what it does to her self-concept and to her feelings for you.
So I suggest you NOT "try to put this behind you" because it could burn inside of you like it has burned inside of your sister for all these years. If you bring it up with her and say you want to get thru your feelings together, so you can be more at peace with what happened than you are now, she'll probably be less frightened of what's going to be said. But there's still no way for her to get around her guilt and shame--And that's where YOU have the power to promote the healing for both of you.
As for your husband, this may be no consolation, but the best nationwide research in around 1990 (I used in my textbook publ 2000) found that about 1/3 of all white men had cheated at least once during the lifetime of their marriages, compared to 19% of white women, but the women's frequency was rising. In my experience with men's support groups (usually college educated) the frequency was less. But most important, almost none of the men had cheated more than once, because the outcome of such an adventure was always much worse and more long-lasting than the excitement of the moment.