Big dilemma!! help

My partner's brother is about to buy a house with his long time girlfriend named S. 
We received a message from her sister (they don't get along at all) who claims S slept with another guy last october and is laughing about it. Should we tell brother or stay out of it? We are feeling so torn her sister could be lying for all we know. 
S will deny it, and brother will believe her. And last thing we want is to create tension in the family. What would you do? Thank you 

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