Brother-in-law secretly cheated on his wife's sister while his wife was pregnant

A wife returns to her parents' house to give birth. I'm living alone for the first time in a long time, and I'm going crazy. My wife's sister came to my house whether I knew it or not. “Brother-in-law… Aren't you trying to deceive?” When I couldn't hide my discomfort from my sister who suddenly followed my core … “I will handle it for you so your brother-in-law doesn't cheat..." hand….