My sister is cheating on her husband

I recently found out that my sister is cheating on her husband of 10 years from a guy from our hometown. I found it out accidentally when I saw them entering a hotel together. I confronted her and she brokedown infront of me saying it was a mistake and she doesn't know herself how it got to that point. 

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My brother in law is a very good husband to her and they have a 5 year old daughter together, he's the most calm and put together man and a loving husband and dad to my niece. 

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I asked her if she did it because there were any problems with them and their marriage and she came out that it's nothing like that and she is very happy in her marriage. 

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For now I didn't say anything to by brother in law and was contemplating what to do, she has said all these things and that she regrets doing it but has not stopped seeing the other guy in secret.

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Will I be the asshole to come clean to my brother in law as I'm feeling bad for that man and my niece and as a man myself  I am completely disgusted with the behaviour of my sister.

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What do I do?

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P.s I'm using a throwaway account as most of my family members know about my reddit account and I'm pretty active on reddit from my main account.

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