Saw my in-law cheating. Do I tell my wife?

Iím in a really hard position right now. I saw my wifeís brother cheating on his wife, and Iím not sure if I should tell her or not. I know for sure she will call him out, despite him being her big brother. I donít them to fight but I also donít like what heís doing to his wife. I already know heís not the best person in the world. In fact, he was bragging about some internet brides that heís chatting with for fun and even asked if I wanted to get to know someone too. Heís really somewhat of a jerk. Do I tell my wife everything even if it means causing trouble between them and her brother?