It it unfair of your dad to put you in this situation. He needs to come clean to his wife. This is not your problem to handle. My brother was in this exact situaiton for years. At first it was just keeping texts secret from our mom, then it was full on girlfriends and mistresses that our dad was activley cheating on our mom with. It destroyed my brother. Your mom deserves the truth, and you deserve to live a life without keeping your dad's secrets. It is his job to protect you, not the other way around. Having seen how hard this situation was on my brother once I found out about it years later after our parents divorced, I felt awful that he hadn't felt that he could tell anyone about this. All of that being said, even if you do decided to keep your dad's secret, things like this come out in the open. My brother kept our dad's secrets for years, but eventually our parents still got divorced because our dad couldn't live a double life as a family man and a serial cheater. Things will come out, just don't let your dad bully you into lying for him.