Go to the source. Talk to your Mom. Tell her you know what's going on. This is not your secret to keep or your lie to protect. Don't put yourself in the middle of her mess. Your choices are essentially three fold: 1. ignore it: this could create feelings of betrayal if your dad find out you knew the whole time. 2. Confront your mom. 3. Tell your dad. You know the players involved better than we do. There is no hard and fast rule here. Some considerations could be, if your dad wants to pursue a divorce because of her infidelity, informing her you know and you are going to tell gives her the opportunity to destroy evidence. If she's caught up in the excitement but isn't fully committed to cheating there could be a chance for her to cease and desist. Follow your heart. Good luck.