it's not quite the same, but I feel you on this one, when I was 18 my mom told me she was going to split up from my dad (I think her intentions were to warn me so I wasn't blind sided but all I felt was guilt). I was told not to say anything to him, he noticed my mom was acting different before she told him and he asked me what was up with her, I had to choose between going against what my mom asked, and lying to my dad's face. definitely not the same as what you're going through but it's a really uncomfortable position to be in for sure.

This is exactly why a wise parent does not get the children involved in the drama between the parents when things are going badly. It’s inherently unfair to say negative things about one parent and not give the other a chance to state their point of view. Really wise parents decide what they are going to do and then together tell the children what is happening without attempting to incriminate one or the other. There are always two sides to every disagreement or conflict - Believe me on this! My parents divorced and remarried and got divorced again and I have divorced also. In every case I know of, even when there was more fault on one side or the other - There is always some merit and fault on each side. Both of them are still your parents and will always be. Your Mom has put you in an unbearable position. Tell her that she must tell Dad about this revelation or you will have to. That is the fairest thing to do. If she doubles down on insisting you remain silent then you know that she is not most concerned about your interests but hers. That relieves you of any obligation to be silent with Dad. When my own parents got divorced, even though the fault was clearly mostly from Mom, my Dad insisted that I treat my mom with love and compassion. Of course we all have different parents. Even the parent who might have done “wrong” has their feelings and reasons for the things they did and those might be quite valid. Try really hard to make yourself neutral in the marriage difficulties your parents are experiencing. Believe your own eyes and memory over what one says about the other and draw your own conclusions. Don’t pile on when one criticizes the other. Always remember that both of them have been your parents. Parents are real people. The best of them try the best they can to be a good mom or dad. Sometimes the relationship between husband and wife, or whatever the relationship is, is not what they want or are able to live with. Life is not some old Disney movie, you are not going to “fix” your parents problems with each other. You can and will adjust to whatever the situation becomes. And both of your parents will still love you.