Husband caught with House girl!!!!

My name is Kiki. I had issues with my husband and moved out of our matrimonial home. I moved into my husband's sister's house (Rekia)who is married because we are very close friends. Rekia travelled with the kids for holiday but Josh, her husband stayed back due to business engagements. On a certain night, I got back from an event and walked in on Josh and the house girl having sex. Both have been begging me not to tell Rekia, my friend and sister Inlaw. What do I do? Should I act like nothing happened? Your opinions are highly solicited.